JPWO2020027017A1 - Manufacturing method of hot-dip galvanized steel sheet and manufacturing method of alloyed hot-dip galvanized steel sheet - Google Patents

Abstract

Provided is a method for manufacturing a hot-dip galvanized steel sheet, which can suppress the occurrence of dross defects even if bottom dross is generated during the hot-dip galvanizing process. The manufacturing method of the present disclosure includes a coarse bottom dross generation step of adjusting the free Al concentration CAl and the bath temperature T in the hot dip galvanizing bath so as to satisfy the formula (1) to generate a coarse bottom dross in the hot dip galvanizing bath. The hot-dip galvanizing treatment is carried out by adjusting the free Al concentration CAl and the bath temperature T of the hot-dip galvanizing bath containing the coarse bottom dross so as to satisfy the formula (2) to form a hot-dip galvanizing layer on the steel sheet. It is equipped with a zinc plating treatment step. 466.15 × CAl + 385.14 ≦ T ≦ 577.24 × CAl + 382.49 (1) 390.91 × CAl + 414.20 ≦ T ≦ 485.00 (2) Here, in the formulas (1) and (2). The free Al concentration CAl (mass%) in the hot-dip galvanizing bath is substituted for "CAL".

Description

The present disclosure relates to a method for producing a hot-dip galvanized steel sheet and a method for producing an 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 methods. First, a steel sheet (base steel sheet) to be hot-dip galvanized is prepared. The base steel plate may be a hot-rolled steel plate or a cold-rolled steel plate. The prepared base steel sheet (the above-mentioned hot-rolled steel sheet or cold-rolled steel sheet) is immersed in a hot-dip galvanized bath, and a hot-dip galvanized treatment is carried out to produce a hot-dip galvanized steel sheet. When the alloyed hot-dip galvanized steel sheet is manufactured, the alloyed hot-dip galvanized steel sheet is further manufactured by heat-treating the hot-dip galvanized steel sheet in an alloying furnace.

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

In the hot-dip galvanizing treatment, for example, an annealed steel sheet is immersed in a hot-dip galvanizing bath. Then, the sink roll arranged in the hot-dip galvanizing bath changes the traveling direction of the steel sheet upward, and pulls the steel sheet out of the hot-dip galvanizing bath. Wiping gas is sprayed from the gas wiping device onto the surface of the steel sheet that is pulled up and advances upward. The wiping gas scrapes off excess molten zinc and adjusts the amount of plating adhered to the surface of the steel sheet. The hot-dip galvanizing treatment is carried out by the above method. In the case of producing an alloyed hot-dip galvanized steel sheet, a steel sheet having an adjusted plating adhesion amount is further charged into an alloying furnace to carry out an alloying treatment.

By the way, in the hot-dip galvanizing treatment described above, Fe is eluted from the steel sheet immersed in the hot-dip galvanizing bath into the hot-dip galvanizing bath. When Fe eluted from the steel sheet in the hot-dip galvanizing bath reacts with Al and Zn existing in the hot-dip galvanizing bath, an intermetallic compound called dross is formed. There are top dross and bottom dross in dross. The top dross is an intermetallic compound having a lighter specific gravity than the hot-dip galvanizing bath, and is a dross that floats on the liquid surface of the hot-dip galvanizing bath. The bottom dross is an intermetallic compound having a heavier specific gravity than the hot-dip galvanizing bath, and is a dross deposited on the bottom of the hot-dip galvanized pot.

During the hot-dip galvanizing process, an accompanying flow is generated due to the progress of the steel sheet in the hot-dip galvanizing bath. The accompanying flow means that a flow is generated in the hot-dip galvanizing bath as the steel sheet progresses. As described above, since the top dross floats on the liquid surface of the hot-dip galvanizing bath, it is not so affected by the accompanying flow. On the other hand, the bottom dross is deposited on the bottom of the hot-dip zinc pot. Therefore, the accompanying flow may wind up from the bottom of the accumulated hot-dip zinc pot. In this case, the bottom dross floats in the hot dip galvanizing bath. Such floating bottom dross may adhere to the surface of the steel sheet during the hot-dip galvanizing process.

The bottom dross adhering to the surface of the steel sheet may cause punctate defects on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. Such surface defects caused by bottom dross are referred to as "dross defects" in the present specification. Dross defects deteriorate the appearance of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet, and reduce the corrosion resistance. Therefore, it is preferable that the occurrence of dross defects can be suppressed.

Techniques for suppressing the occurrence of dross defects have been proposed in Japanese Patent Application Laid-Open No. 11-35096 (Patent Document 1) and Japanese Patent Application Laid-Open No. 11-35097 (Patent Document 2).

In Patent Document 1, in the method for producing an alloyed hot-dip galvanized steel sheet, the hot-dip galvanized bath temperature is T (° C.), and the boundary Al concentration defined by Cz = −0.015 × T + 0.76 is Cz (wt%). And. In this case, the hot-dip zinc bath 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 ± 0.01 wt%. 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 method for producing an alloyed hot-dip galvanized steel sheet, the Al concentration in the bath is kept within the range of 0.15 ± 0.01 wt%. In Patent Document 2, hot-dip galvanizing is performed at the boundary between the top dross and the δ _{1 phase.}

Japanese Patent Application Laid-Open No. 11-35096 Japanese Unexamined Patent Publication No. 11-350097

Plastic Applications of Phase Diagrams in Constant Galvanizing, Nai-Yong Tang, Journal of Phase Equation and Difference Vol. 27 No. 5,2006

As described in Patent Documents 1 and 2, it is known that when the Al concentration in the hot-dip galvanizing bath is increased, most of the dross becomes top dross instead of bottom dross. The top dross floats on the liquid surface of the hot-dip galvanizing bath. Therefore, removing the top dross from the hot-dip galvanizing bath is easier than removing 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 increased, and the dross in the hot-dip galvanizing bath is floated and removed on the liquid surface of the hot-dip galvanizing bath as the top dross, resulting in dross defects. In some cases, a method of suppressing the occurrence of In this specification, the operation of generating the top dross as the dross is referred to as the top dross operation.

In top dross operation, dross defects can be suppressed. However, if the Al concentration in the hot-dip galvanizing bath is increased, the hot-dip galvanizing layer becomes difficult to alloy in the alloying treatment. Therefore, in order to promote alloying, it is particularly preferable to suppress the Al concentration in the hot-dip galvanizing solution. When the top dross operation is carried out, the Al concentration in the hot-dip galvanizing bath is inevitably high.

An operation in which the Al concentration in the hot-dip galvanizing bath is suppressed and bottom dross is generated as dross is referred to as a bottom dross operation in this specification. In the case of bottom dross operation, alloying can be promoted because the free Al concentration in the hot-dip galvanizing bath is suppressed. However, in the case of bottom dross operation, a method capable of suppressing dross defects due to the generation of bottom dross is required.

An object of the present invention is to provide a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet, which can suppress the occurrence of dross defects even if bottom dross is generated during the hot-dip galvanizing process.

The method for manufacturing a hot-dip galvanized steel sheet according to the present disclosure is as follows.
_{The free Al concentration C Al} (mass%) and the bath temperature T (° C.) in the hot-dip galvanizing bath are adjusted so as to satisfy the formula (1), and the coarse bottom dross having a particle size of 300 μm or more is adjusted in the hot-dip galvanizing bath. Coarse bottom dross generation process and
_{The free Al concentration C Al} and the bath temperature T of the hot-dip galvanizing bath after the coarse bottom dross generation step are adjusted so as to satisfy the formula (2), and the free Al concentration C _Al and the bath temperature T are expressed by the formula ( The hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on a steel sheet by performing a hot-dip galvanizing treatment using the hot-dip galvanizing bath satisfying 2) is provided.
466.15 × C _Al +385.14 ≦ T ≦ 577.24 × C _Al +382.49 (1)
390.91 × C _Al +414.20 ≦ T ≦ 485.00 (2)

The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present disclosure is as follows.
A process of manufacturing a hot-dip galvanized steel sheet by carrying out the above-mentioned method for manufacturing a hot-dip galvanized steel sheet,
The hot-dip galvanized steel sheet is provided with an alloying treatment step of performing an alloying treatment.

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

FIG. 1 is a metastable state diagram in which the dross-forming phase of the hot-dip galvanizing bath is _{arranged 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 the hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet used for manufacturing the hot-dip galvanized steel sheet. FIG. 3 is a side view of the hot-dip galvanizing facility in FIG. FIG. 4 is a side view of a hot-dip galvanizing facility having a configuration different from that of FIG. FIG. 5 is a side view of a hot-dip galvanizing facility having a configuration different from that of FIGS. 3 and 4. FIG. 6 is a functional block diagram showing the overall configuration of the hot-dip galvanizing line equipment having a configuration different from that of FIG. FIG. 7 is a schematic diagram for explaining a method for measuring the particle size of the bottom dross. FIG. 8 is a photomicrograph showing the morphology of the bottom dross formed in the hot-dip galvanized bath 10 days after the construction bath in Examples 1 and 2. FIG. 9 is a graph showing the relationship between the particle size and the number of dross under each production condition of Example 5.

[Causes of dross defects]
The present inventors first examined the dross that causes dross defects when the bottom dross operation is carried out. Dross defects are caused by dross generated during hot-dip galvanizing. In previous studies, it has been reported that there are the following types of dross generated in hot-dip galvanizing treatment.
(A) Top dross (B) δ _1- phase dross (C) Γ _1- phase dross (D) ζ-phase dross

As mentioned above, the top dross has a lighter specific density than the hot-dip galvanizing bath. Therefore, the top dross easily floats on the liquid surface of the hot-dip galvanizing bath. The chemical composition of top dross is 45% Al, 38% Fe and 17% Zn in mass%. Since the top dross floats on the liquid surface of the hot-dip galvanizing bath, it can be easily removed from the hot-dip galvanizing bath. Therefore, when the top dross is operated, the dross defect can be effectively suppressed by removing the top dross.

On the other hand, δ _1- phase dross, Γ _1- phase dross, and ζ-phase dross are called bottom dross. The bottom dross has a heavier specific gravity than the hot-dip galvanized bath. Therefore, the bottom dross tends to be deposited on the bottom of the hot-dip galvanized pot in which the hot-dip galvanizing bath is stored. Previous studies have suggested that these bottom dross contribute to dross defects when performing bottom dross operations.

Here, the crystal structure of the δ _1- phase dross is hexagonal. The chemical composition of the δ _1- phase dross is 1% or less of Al, 9% or more of Fe, and 90% or more of Zn in mass%. The crystal structure of the gamma _1-phase dross is face-centered cubic crystal. The chemical composition of the gamma ₁ phase dross, in mass%, and 20% Fe, consisting of about 80% Zn. The crystal structure of the ζ-phase dross is monoclinic. The chemical composition of the ζ phase is 1% or less of Al, about 6% Fe, and about 94% Zn in mass%.

In previous studies, there were many reports of bottom dross in which the main cause of dross defects was δ _{1 phase dross.} In the above-mentioned Patent Documents 1 and 2, it seems that the δ _1- phase dross is considered to be one of the causes of the dross defect. Therefore, the present inventors also initially considered that δ _1- phase dross was the main cause of dross defects, and conducted investigations and studies. _{However, even when the occurrence of δ 1-} phase dross was suppressed in the hot-dip galvanizing treatment, dross defects may still occur on the surfaces of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet.

Therefore, the present inventors considered that the cause of the dross defect _{was not the δ 1-} phase dross but another dross. Therefore, the present inventors carried out a bottom dross operation using an alloyed hot-dip galvanized steel sheet in which dross defects were generated, and analyzed the composition and crystal structure of the dross defects portion again. The present inventors further analyzed the types of dross generated in the hot-dip galvanizing bath in the bottom dross operation. As a result, the present inventors have obtained the following findings regarding the dross defect, which are different from the results of conventional studies.

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

_{The chemical composition of δ 1-} phase dross (Al of 1% or less by mass, Fe of 9% or more, and Zn of 90% or more), which was considered to be the main cause of conventional dross defects, is the above-mentioned dross defect portion. Similar to the chemical composition of. However, _{the crystal structure of the δ 1-} phase dross is hexagonal, not the face-centered cubic identified in the dross defect portion. Therefore, the present inventors considered that the δ _1- phase dross, which was conventionally considered to be the main cause of the dross defect, is not actually the main cause of the dross defect.

Therefore, the present inventors have identified the dross that causes the dross defect. Of dross above bottom dross (B) ~ (D), for the gamma _1-phase dross, although the crystal structures are the same face-centered cubic dross defect, with the chemical composition (mass% 20% Fe, and , 80% Zn) is significantly different from the chemical composition of the dross defect portion. Regarding the ζ-phase dross, the chemical composition (Al of 1% or less in mass%, Fe of about 6%, and Zn of about 94%) is different from the chemical composition of the dross defect portion, and further, the crystal structure (monoclinic crystal). ) Is also different from the crystal structure (face-centered cubic crystal) of the dross defect part.

Based on the above examination results, the present inventors considered that the dross defect was not caused by the dross of (B) to (D) described above. Then, the present inventors considered that the dross defect may be caused by other types of bottom dross other than the above (B) to (D).

Therefore, the present inventors further analyzed the bottom dross in the hot-dip galvanizing bath. The above-mentioned EPMA and TEM were used for the analysis of bottom dross. As a result, the present inventors have _{found that Γ 2-} phase dross exists as the bottom dross generated in the hot-dip galvanizing bath.

The chemical composition of the gamma ₂ phase dross, in mass%, and 2% of Al, 8% Fe, consists of 90% of Zn, consistent with the chemical composition of the analyzed dross defect portion above. Further, the crystal structure of the gamma ₂ phase dross is face-centered cubic, consistent with the crystal structure of the dross defect. Accordingly, the present inventors have, gamma ₂ phase dross thought that it is the main factor of dross defects. Since the specific gravity of the gamma ₂ phase dross is greater than the specific gravity of the molten zinc plating bath, gamma ₂ phase dross was true dross which may deposit on the bottom of the molten zinc pot.

The present inventors have found that the gamma ₂ phase dross, regarding the dross other (B) ~ (D), further investigating. As a result, it was found that the dross defect is caused by the hard dross, and the soft dross is difficult to form the dross defect. Results of further studies by the present inventors, the dross of the (B) ~ (D), and, among the gamma ₂ phase dross, it was found gamma ₂ phase dross is dross hardest.

Based on the above examination results, the present inventors have found that the main cause of dross defects generated on the surfaces of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets to be hot-dip galvanized is δ _1- phase dross. Instead, I thought it was a _{Γ 2-phase dross.} Furthermore, the present inventors consider that the dross classified as bottom dross is one of Γ _2- phase dross, δ _1- phase dross, ζ-phase dross, and Γ _1- phase dross, but in the hot-dip galvanizing bath, Γ. _{It was found that one-} phase dross is almost nonexistent.

Therefore, the present inventors further investigated the dross defect when the bottom dross operation was carried out while suppressing the free Al concentration in the hot-dip galvanizing bath. The accompanying flow generated by passing the steel plate through the hot-dip galvanizing bath winds up a part of the bottom dross accumulated at the bottom of the hot-dip galvanized pot. Then, the rolled-up bottom dross adheres to the steel sheet. In this case, dross defects can occur.

Here, the present inventors focused on the size of the bottom dross and proceeded with the study. As a result, the present inventors obtained the following findings. Bottom dross with a particle size of less than 100 μm is defined as fine bottom dross. The fine bottom dross can be rolled up from the bottom of the hot-dip galvanized pot into the hot-dip galvanizing bath by accompaniment. However, even if the fine bottom dross is wound up and adheres to the steel sheet, the size of the bottom dross is small, so that dross defects are unlikely to occur. On the other hand, a bottom dross having a particle size of more than 300 μm is defined as a coarse bottom dross. The mass of the oversized bottom dross is heavy. Therefore, the coarse bottom dross is less likely to be wound up by the accompanying flow and is less likely to adhere to the steel sheet. As a result of the above investigation, the present inventors have found that the bottom dross that causes dross defects is a bottom dross having a particle size of 100 to 300 μm (hereinafter referred to as medium-sized bottom dross).

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

First, the present inventors focused on the growth rate of each bottom dross in order to suppress the formation of medium-sized bottom dross. _{Of the bottom dross and Γ 2-} phase dross of (B) to (D) above _{, the Γ 2-} phase dross is the fastest and the δ _1- phase dross is the slowest. Thus, gamma ₂ phase dross grows faster than [delta] ₁ phase dross, the particle size of the gamma ₂ phase dross at a much earlier stage than [delta] ₁ phase dross exceeds 100 [mu] m. In contrast, the growth rate of _{δ 1-} phase dross is significantly slower than that of _{Γ 2-phase dross.} Therefore, even if δ ₁ phase dross is nucleated, δ ₁ phase dross is less likely to grow as early as _{Γ 2 phase.} Therefore, during the period (operating period) during which the hot-dip galvanizing treatment step is carried out, it is considered preferable to carry out the hot-dip galvanizing treatment in _{the δ 1-} phase forming region rather than the _{Γ 2-phase dross forming region.}

Therefore, the present inventors further investigated and examined the bath 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 dross. rice field. As a result, the present inventors created a metastable phase diagram of dross in the hot-dip galvanized bath shown in FIG. Hereinafter, FIG. 1 will be described.

The vertical axis of FIG. 1 shows the free Al concentration C _Al (mass%) in the hot-dip galvanizing bath. Here, in the present specification, the "free Al concentration C _Al in the hot-dip galvanizing bath" means the free Al concentration (mass%) melted in the hot-dip galvanizing bath. That is, in the present specification, the "free Al concentration C _Al in the hot-dip galvanizing bath" is melted in the hot-dip galvanizing bath excluding the Al content contained in the dross (top dross and bottom dross). It means the free Al concentration (in other words, in the liquid phase) (% by mass). The horizontal axis of FIG. 1 indicates the bath temperature T (° C.) in the hot-dip galvanizing bath.

With reference to FIG. 1, in the free Al concentration C _Al range and the bath temperature T (° C.) shown in FIG. , There is a region 2 where the Γ _2- phase dross is generated (hereinafter referred to as the Γ _2- phase generation region 2) and _{a region 3 where the δ 1-} phase dross is generated (hereinafter referred to as the δ _1- phase generation region 3).

The top dross generation region 1A and gamma ₂ phase generation region 2, are defined by the transformation line F _12. The top dross generation region 1A and [delta] _1-phase generation region 3 are partitioned by the transformation line F _13. The Γ _2- phase generation region 2 and the δ _1- phase generation region 3 are separated by the transformation line F ₂₃ .

For example, in a hot-dip galvanizing bath in which the bath temperature T is 440 ° C. and the free Al concentration C _{Al is 0.135%, Γ 2-} phase dross is generated. It is assumed that the bath temperature T is raised from 440 ° C. to 470 ° C. while maintaining _{the free Al concentration C Al} of this hot-dip galvanizing bath at 0.135%. In this case, the state of the hot-dip galvanizing bath _{shifts from the Γ 2} phase generation region 2 to the δ ₁ phase generation region 3 _{beyond the transformation line F 23. Therefore, the Γ 2-} phase dross in the hot-dip galvanizing bath undergoes a phase transformation to become a δ _1- phase dross. _{Further, it is assumed that the free Al concentration C Al} of the hot-dip galvanizing bath in which the bath temperature T is 440 ° C. and the free Al concentration C _Al is 0.135% is increased to 0.140%. In this case, the state of the galvanizing bath, beyond the transformation line F ₁₂ from gamma ₂ phase generation region 2, moves to the top dross generation region 1A. _{Therefore, the Γ 2-} phase dross in the hot-dip galvanizing bath undergoes phase transformation to become the top dross.

Furthermore, the present inventors further have a boundary line F ₂₁₂₂ that _{separates the Γ 2} nucleation region 21 and the Γ _{2 grain growth region 22 in the Γ 2} phase generation region 2 of the metastable state diagram shown in FIG. Was found. Furthermore, the present inventors further have a boundary line F ₃₁₃₂ that _{separates the δ 1} nucleation region 31 and the δ _{1 grain growth region 32 in the δ 1} phase generation region 3 of the metastable state diagram shown in FIG. Was found. This point will be described below.

In gamma ₂ phase generation region 2, gamma ₂ nucleation region 21, than the gamma ₂ grain growth region 22 located on the cold side of the boundary line F _2122. In the Γ ₂ nucleation region 21, nucleation _{of Γ 2-} phase dross is promoted in the hot dip galvanizing bath as compared with _{the Γ 2 grain growth region 22.} That is, _{the formation of fine Γ 2-} phase dross is promoted. On the other hand, the gamma ₂ grain growth region 22, as compared to the gamma ₂ nucleation region 21, the growth of the gamma ₂ phase already present in the molten zinc plating bath (grain growth) is promoted.

Similarly, in the δ ₁ phase generation region 3, the δ ₁ nucleation region 31 is located on the high temperature side of the _{boundary line F 3132 with respect to the δ 1} grain growth region 32. In the δ ₁ nucleation region 31, nucleation _{of δ 1} phase dross is promoted in the hot dip galvanizing bath as compared with _{the δ 1 grain growth region 32.} That is, _{the formation of fine δ 1-} phase dross is promoted. On the other hand, in the δ ₁ grain growth region 32, the growth (grain growth) of _{the δ 1} phase already existing in the hot-dip galvanizing bath _{is promoted as compared with the δ 1 nucleation region 31.}

_{The transformation line F 23} in the metastable phase diagram can be defined by the following equation (A).
F ₂₃ = 577.24 x C _Al +382.49 (A)
Further, the boundary line F ₂₁₂₂ can be defined by the following equation (B).
F ₂₁₂₂ = 466.15 x C _Al +385.14 (B)
Further, the boundary line F ₃₁₃₂ can be defined by the following equation (C).
F ₃₁₃₂ = 390.91 x C _Al +414.20 (C)
_{Here, the free Al concentration C Al} (mass%) in the hot-dip galvanizing bath _{is substituted for "C Al} " in the formulas (A) to (C).

Based on the metastable phase diagram of FIG. 1, the present inventors have studied a hot-dip galvanizing treatment method capable of suppressing dross defects. As mentioned above, the dross defect is caused by a medium-sized bottom dross having a particle size of less than 100 to 300 μm. In order to suppress the formation of medium-sized bottom dross during the hot-dip galvanizing treatment (during the operating period), the hot-dip galvanizing bath should be in the δ ₁ nucleation region 31 in FIG. , The free Al concentration C _Al and the bath temperature T in the hot-dip galvanizing bath may be adjusted. In the δ ₁ nucleation region 31, nucleation of δ ₁ phase dross is promoted, but _{growth of δ 1} phase dross is suppressed. Furthermore, as described above, among the bottom _{dross, the δ 1} phase dross has the slowest growth rate. Therefore, if the hot-dip galvanizing bath during the operating period is set to the δ ₁ nucleation region 31, the bottom _{dross (here, the δ 1} phase dross) can be suppressed from growing into a medium-sized bottom dross.

However, even if the hot-dip galvanizing bath is _{maintained in the δ 1} nucleation region 31 during the operating period, if the hot-dip galvanizing treatment is carried out for a long period of time (operating period), fine δ _1- phase dross also grows to some extent. do. Therefore, if the hot-dip galvanizing bath during the operating period is simply set to the δ ₁ nucleation region 31, medium-sized bottom dross may be generated if the operating period is long.

When the medium-sized bottom dross increases in the hot-dip galvanizing bath, the occurrence of dross defects can be suppressed by performing the work of removing the bottom dross from the hot-dip zinc pot (hereinafter referred to as the bottom dross removing step). However, when the bottom dross removal step is carried out, the hot dip galvanizing process must be stopped and the continuous hot dip galvanizing line equipment must be stopped. The state in which such hot-dip galvanizing treatment is stopped is referred to as "stop" in the present specification. When the above-mentioned bottom dross removing step is adopted, the higher the frequency of the bottom dross removing step, the lower the production efficiency.

Therefore, the present inventors have further investigated a method capable of sufficiently suppressing dross defects while suppressing the frequency of the bottom dross removing step when performing the hot-dip galvanizing treatment in the bottom dross operation. As a result, the present inventors intentionally impregnate the hot-dip galvanizing bath with a coarse bottom dross having a particle size of 300 μm or more, and then carry out the hot-dip galvanizing treatment in _{the δ 1 nucleation region 31.} It was found that the formation of medium-sized bottom dross can be suppressed over a period of time. This point will be described below.

When the bottom dross reaches a certain size, it grows due to Ostwald growth. Ostwald ripening means that when metal particles of the same type having different particle sizes are present in the matrix phase (Zn in the liquid phase in the present specification), the metal particles having a small particle size shrink or disappear, and the metal particles having a large particle size shrink or disappear. Is a phenomenon in which the particles grow even more coarsely.

In the present embodiment, the hot-dip galvanizing bath before the hot-dip galvanizing treatment is previously impregnated with a coarse bottom dross having a particle size of 300 μm or more. Then, a hot-dip galvanizing treatment is carried out in the _{δ 1} nucleation region 31 using a hot-dip galvanizing bath containing a coarse bottom dross. In this case, during the hot-dip galvanizing treatment, _{nucleated δ 1-} phase dross occurs, but the nucleated δ _1- phase dross shrinks or disappears and is absorbed by the coarse bottom dross (in this case, coarse δ _1- phase dross). Will be done. That is, when a hot-dip galvanizing bath containing a coarse bottom _{dross is used, the fine δ 1-} phase dross is absorbed by the coarse bottom _{dross (coarse δ 1-} phase dross) by Ostwald ripening. Even if the coarse bottom dross grows further, the mass of the coarse bottom dross is large, so that it is difficult to wind up due to the accompanying flow. In the case of this method, even if the hot-dip galvanizing treatment is carried out for a long period of time _{, the formation of fine δ 1-} phase dross can be suppressed. Therefore, the generation of medium-sized bottom dross is further suppressed. As a result, it is possible to suppress the occurrence of dross defects even if the operating period is long.

In short, in the present embodiment, after the coarse bottom dross is previously contained in the hot-dip galvanizing bath, the hot-dip galvanizing treatment is carried out in the _{δ 1 nucleation region 31.} In this case, the coarse bottom dross further grows due to the Ostwald growth, so _{that not only the growth of the fine bottom dross (fine δ 1-} phase dross) but also the formation can be suppressed. As a result, the generation of medium-sized bottom dross, which causes dross defects, can be effectively suppressed.

As described above, in the method for producing a hot-dip galvanized steel sheet of the present embodiment, the hot-dip galvanizing treatment is carried out in a state where the coarse bottom dross, which was conventionally considered to be a target to be removed, is intentionally included in the hot-dip galvanizing bath. It was completed based on a completely different technical idea. Specifically, the method for producing the hot-dip galvanized steel sheet of the present embodiment is as follows.

The method for manufacturing the hot-dip galvanized steel sheet of [1] is as follows.
_{A coarse bottom dross generation step of adjusting the free Al concentration C Al} and the bath temperature T in the hot dip galvanizing bath so as to satisfy the formula (1) to generate a coarse bottom dross having a particle size of 300 μm or more in the hot dip galvanizing bath. When,
_{The free Al concentration C Al} and the bath temperature T of the hot-dip galvanizing bath after the coarse bottom dross generation step are adjusted so as to satisfy the formula (2), and the free Al concentration C _Al and the bath temperature T are expressed by the formula ( The hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on a steel sheet by performing a hot-dip galvanizing treatment using the hot-dip galvanizing bath satisfying 2) is provided.
466.15 × C _Al +385.14 ≦ T ≦ 577.24 × C _Al +382.49 (1)
390.91 × C _Al +414.20 ≦ T ≦ 485.00 (2)

The method for manufacturing the hot-dip galvanized steel sheet in [2] is as follows.
The method for producing a hot-dip galvanized steel sheet according to [1].
When the operation of the hot-dip galvanizing treatment step is stopped, the coarse bottom dross generation step is carried out on the hot-dip galvanizing bath after the hot-dip galvanizing treatment step.

The method for manufacturing the hot-dip galvanized steel sheet in [3] is as follows.
The method for producing a hot-dip galvanized steel sheet according to [1] or [2].
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 obtain the hot-dip galvanizing bath satisfying the formula (2).

The method for manufacturing the hot-dip galvanized steel sheet in [4] is as follows.
The method for producing a hot-dip galvanized steel sheet according to [3].
The coarse bottom dross generation step and the hot-dip galvanizing treatment step are alternately repeated.
When the coarse bottom dross generation step is carried out after the hot-dip galvanizing treatment step, in the coarse bottom dross step, the bath temperature T of the hot-dip galvanizing bath after the hot-dip galvanizing treatment step is lowered to obtain the formula (1). ) Satisfying the above-mentioned hot-dip galvanizing bath.

The method for manufacturing the hot-dip galvanized steel sheet in [5] is as follows.
The method for producing a hot-dip galvanized steel sheet according to any one of [1] to [4].
_{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 0.125% by mass or more.

The method for manufacturing the hot-dip galvanized steel sheet in [6] is as follows.
The method for producing a hot-dip galvanized steel sheet according to [5].
_{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 set to 0.138% by mass or less.

The method for manufacturing the hot-dip galvanized steel sheet in [7] is as follows.
The method for producing a hot-dip galvanized steel sheet according to any one of [1] to [6].
Before carrying out the coarse bottom dross generation step, a bottom dross removing step of removing at least a part of the coarse bottom dross in the hot-dip galvanizing bath is provided.

The method for manufacturing the alloyed hot-dip galvanized steel sheet in [8] is as follows.
A step of manufacturing the hot-dip galvanized steel sheet by carrying out the method for manufacturing the hot-dip galvanized steel sheet according to any one of [1] to [7].
The hot-dip galvanized steel sheet is provided with an alloying treatment step of performing an alloying treatment.

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

[Structure of hot-dip galvanizing line equipment]
FIG. 2 is a functional block diagram showing an example of the overall configuration of the hot-dip galvanized line equipment used for manufacturing the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet. With reference to FIG. 2, the hot-dip galvanizing line equipment 1 includes an annealing furnace 20, a hot-dip galvanizing equipment 10, and a tempering rolling mill (skin pass mill) 35.

The annealing furnace 20 includes one or more heating zones (not shown) and one or more cooling zones arranged downstream of the heating zones. In the annealing furnace 20, the steel sheet is supplied to the heating zone of the annealing furnace 20, and the steel sheet is annealed. The annealed steel sheet is cooled in the cooling zone and transported to the hot dip galvanizing facility 10. The hot-dip galvanizing facility 10 is located downstream of the annealing furnace 20. In the hot-dip galvanizing facility 10, a hot-dip galvanizing treatment is performed on the steel sheet to produce an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. The tempering rolling mill 35 is arranged downstream of the hot dip galvanizing facility 10. In the tempering rolling mill 35, the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet manufactured in the hot-dip galvanized facility 10 is lightly reduced as necessary, and the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is melted. Adjust the surface of the galvanized steel sheet.

[About hot-dip galvanizing equipment 10]
FIG. 3 is a side view of the hot-dip galvanizing facility 10 in FIG. With reference to FIG. 3, the hot-dip galvanizing facility 10 includes, for example, a hot-dip zinc pot 101, a sink roll 107, a support roll 113, a gas wiping device 109, and an alloying furnace 111.

The inside of the annealing furnace 20 provided in front of the hot-dip galvanizing facility 10 is maintained in a reducing atmosphere. In the annealing furnace 20, the steel sheet S that is continuously conveyed is heated. Heating of the steel sheet S in the annealing furnace 20 activates the surface of the steel sheet S and adjusts the mechanical properties of the steel sheet S. The downstream end of the annealing furnace 20, which corresponds to the exit side of the annealing furnace 20, has a space in which the turndown roll 30 is arranged. The downstream end of the annealing furnace 20 is connected to the upstream end of the snout 105. The downstream end of the snout 105 is immersed in the hot dip galvanizing bath 103. The inside of the snout 105 is shielded from the atmospheric atmosphere and is maintained in a reducing atmosphere.

The steel sheet S whose transport direction has been changed downward by the turndown roll 30 passes through the snout 105 and is continuously immersed in the hot-dip galvanizing bath 103 stored in the hot-dip zinc pot 101. A sink roll 107 is arranged inside the hot-dip zinc pot 101. The sink roll 107 has a rotation axis parallel to the width direction of the steel plate S. The axial width of the sink roll 107 is larger than the width of the steel plate S. The sink roll 107 comes into contact with the steel plate S and changes the traveling direction of the steel plate S to the upper side of the hot dip galvanizing equipment 10.

The support roll 113 is a well-known member. The support roll 113 is arranged in the hot dip galvanizing bath 103 and above the sink roll 107. The support roll 113 includes a pair of rolls. The pair of rolls of the support roll 113 has a rotation axis parallel to the width direction of the steel plate S. The support roll 113 supports the steel plate S to be conveyed upward with the steel plate S whose traveling direction is changed upward by the sink roll 107.

The gas wiping device 109 is arranged above the sink roll 107 and the support roll 113 and above the liquid level of the hot-dip galvanizing bath 103. The gas wiping device 109 includes a pair of gas injection devices. The pair of gas injection devices has gas injection nozzles that oppose each other. During the hot-dip galvanizing process, the steel plate S passes between the pair of gas injection nozzles of the gas wiping device 109. At this time, the pair of gas injection nozzles face the surface of the steel plate S. The gas wiping device 109 blows gas onto both surfaces of the steel sheet S pulled up from the hot-dip galvanizing bath 103. As a result, the gas wiping device 109 scrapes off a part of the hot-dip galvanizing adhering to both surfaces of the steel sheet S, and adjusts the amount of hot-dip galvanizing adhering to the surface of the steel sheet S.

The alloying furnace 111 is arranged above the gas wiping device 109. The alloying furnace 111 passes the steel plate S conveyed upward through the gas wiping device 109 through the inside, and performs the alloying treatment on the steel plate S. The alloying furnace 111 includes a heating zone, a tropical zone, and a cooling zone in this order from the entry side to the exit side of the steel sheet S. The heating band is heated so that the temperature (plate temperature) of the steel plate S becomes substantially uniform. The tropical zone maintains the plate temperature of the steel plate S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to become an alloyed hot-dip galvanized layer. The cooling zone cools the steel sheet S on which the alloyed hot-dip galvanized layer is formed. As described above, the alloying furnace 111 carries out the alloying treatment using the heating zone, the tropical zone, and the cooling zone. The alloying furnace 111 carries out the above-mentioned alloying treatment when producing an alloyed hot-dip galvanized steel sheet. On the other hand, when producing a hot-dip galvanized steel sheet, the alloying furnace 111 does not carry out the alloying treatment. In this case, the steel plate S passes through the non-operating alloying furnace 111. Here, "not operating" means, for example, a state in which the power supply is stopped (a state in which the alloying furnace 111 is not started) while the alloying furnace 111 is placed online. The steel plate S that has passed through the alloying furnace 111 is conveyed to the next process by the top roll 40.

When producing a hot-dip galvanized steel sheet, the alloying furnace 111 may be moved offline as shown in FIG. In this case, the steel plate S is conveyed to the next process by the top roll 40 without passing through the alloying furnace 111.

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

[About other configuration examples of hot-dip galvanizing line equipment]
The hot-dip galvanizing line equipment 1 is not limited to the configuration shown in FIG. For example, when a Ni pre-plating process is performed on a steel sheet before hot-dip galvanizing to form a Ni layer on the steel sheet, Ni is placed between the annealing furnace 20 and the hot-dip galvanizing facility 10 as shown in FIG. The pre-plating equipment 45 may be arranged. The Ni pre-plating equipment 45 includes a Ni plating cell for storing a Ni plating bath. The Ni pre-plating process is carried out by an electroplating method. The hot-dip galvanizing line equipment 1 of FIGS. 2 and 6 includes an annealing furnace 20 and a tempering rolling mill 35. However, the hot dip galvanizing line equipment 1 does not have to include the annealing furnace 20. Further, the hot-dip galvanizing line equipment 1 does not have to be equipped with a tempering rolling mill 35. The hot-dip galvanizing line equipment 1 may be provided with at least the hot-dip galvanizing equipment 10. The annealing furnace 20 and the tempering rolling mill 35 may be arranged as needed. Further, the hot-dip galvanizing line equipment 1 may be provided with a pickling equipment for pickling the steel sheet upstream of the hot-dip galvanizing equipment 10, and equipment other than the annealing furnace 20 and the pickling equipment. May be provided. The hot-dip galvanizing line equipment 1 may be further provided with equipment other than the tempering rolling mill 35 downstream of the hot-dip galvanizing equipment 10.

[About the manufacturing method of the hot-dip galvanized steel sheet of this embodiment]
[About hot-dip galvanizing line equipment to be used]
In the hot-dip galvanizing treatment method of the present embodiment, the hot-dip galvanizing line equipment 1 is used. The hot-dip galvanizing line equipment 1 has, for example, the configurations shown in FIGS. 2 and 6. As described above, the hot-dip galvanizing line equipment 1 used in the hot-dip galvanizing treatment method of the present embodiment may be the equipment shown in FIGS. 2 and 6, and is further added to the equipment shown in FIGS. 2 and 6. Other configurations may be added. Further, as described above, the hot dip galvanizing line equipment 1 does not have to be provided with the annealing furnace 20. Further, the hot-dip galvanizing line equipment 1 does not have to be equipped with a tempering rolling mill 35. The hot-dip galvanizing line equipment 1 may be provided with at least the hot-dip galvanizing equipment 10. A well-known hot-dip galvanizing line facility 1 having a configuration different from that of FIGS. 2 and 6 may 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 steel plate) used in the hot-dip galvanizing treatment method of the present embodiment are not particularly limited. The steel sheet is an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet according to each mechanical property (for example, tensile strength, workability, etc.) required for the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet to be manufactured. A known steel plate applied to the above may be used. The steel sheet used for the outer panel of an automobile may be used as the steel sheet to be hot-dip galvanized. The steel sheet (base 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 further contains Al in addition to Zn. That is, the hot-dip galvanizing bath used in the hot-dip galvanizing treatment method of the present embodiment is a plating solution containing Al having a specific concentration and the balance being Zn and impurities. The impurity is, for example, Fe. If the hot-dip galvanizing bath contains a specific concentration of Al, an excessive reaction between Fe and Zn in the bath can be suppressed. As a result, the progress of the non-uniform alloy reaction between the steel sheet immersed in the hot-dip galvanizing bath and Zn can be suppressed.

As shown in FIG. 1, the preferable lower limit of _{the free Al concentration C Al is 0.125%.} When the free Al concentration C _Al is 0.125% by mass or more, excessive progress of alloying of the hot-dip galvanized layer can be suppressed in the alloying treatment. Therefore, embrittlement of the alloyed hot-dip galvanized layer due to excessive alloying can be suppressed. As a result, the adhesion of the alloyed hot-dip galvanized layer to the steel sheet is improved. The lower limit of the free Al concentration C _Al is more preferably 0.127%, further preferably 0.129%, still 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, the alloying is carried out more effectively, and the alloyed hot-dip galvanized layer is sufficiently formed. A more preferable upper limit of the free Al concentration C _Al is 0.137%, more preferably 0.136%, still more preferably 0.135%.

The free Fe concentration in the hot-dip galvanizing bath 103 is not particularly limited. The free Fe concentration is, for example, 0.020 to 0.060% by mass. The Fe in the hot-dip galvanizing bath 103 may be eluted from the steel plate S, or may be contained in the hot-dip galvanizing bath 103 for another reason. The hot-dip galvanizing bath 103 may contain impurities other than Fe. The term “impurities” as used herein means components that are mixed due to raw materials or other factors and are allowed as long as they do not adversely affect the production method according to the present embodiment.

[Measuring method of free Al concentration and free Fe concentration in hot dip galvanizing bath 103]
The method for determining the free Al concentration and the free Fe concentration in the hot-dip galvanizing bath 103 is not particularly limited. _{For example, the free Al concentration C Al} (mass%) and the free Fe concentration (mass%) are determined based on the Al concentration and Fe concentration obtained by inductively coupled plasma (ICP) emission spectroscopic analysis.

Specifically, a sample is taken from the hot-dip galvanizing bath 103. Quench the sample to solidify. Al concentration and Fe concentration are obtained by ICP emission spectroscopic analysis using the solidified sample. The Al concentration obtained by ICP emission spectroscopic analysis includes not only the free Al concentration in the hot-dip galvanizing bath but also the Al concentration in the dross. That is, the Al concentration obtained by ICP emission spectroscopic analysis is the so-called total Al concentration. Similarly, the Fe concentration obtained by the above-mentioned ICP emission spectroscopic analysis includes not only the free Fe concentration in the hot-dip galvanizing bath but also the Fe concentration in the dross. That is, the Fe concentration obtained by ICP emission spectroscopic analysis is the so-called total Fe concentration. _{Therefore, the free Al concentration C Al} and the free Fe concentration are determined by using the obtained total Al concentration and total Fe concentration and a well-known Zn-Fe-Al ternary phase diagram.

The method for determining the free Al concentration C _Al and the free Fe concentration is as follows. Prepare a Zn-Fe-Al ternary phase diagram at the bath temperature T when the sample is taken. As described above, the Zn-Fe-Al ternary phase diagram is well known and is also disclosed in FIGS. 2 and 3 in Non-Patent Document 1. Non-Patent Document 1 is a well-known paper among researchers and developers of hot-dip galvanizing baths. On the Zn-Fe-Al ternary phase diagram, the points specified from the total Al concentration and the total Fe concentration obtained by the ICP emission spectroscopy are plotted. Then, a tie line (conjugate line) is drawn on the liquid phase line in the Zn-Fe-Al ternary system state diagram from the plotted points. The Al concentration at the intersection of the liquidus line and the tie line is _{defined as the free Al concentration C Al,} and the Fe concentration at the intersection of the liquid phase line and the tie 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 obtained. Of the chemical composition of the hot-dip galvanizing bath, _{the balance 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 method for producing a hot-dip galvanized steel sheet according to the present embodiment includes a coarse bottom dross generation step (S1) and a hot-dip galvanized steel sheet treatment step (S2). Hereinafter, each step will be described.

[Coarse bottom dross generation step (S1)]
The coarse bottom dross generation step (S1) is carried out during a period in which the hot-dip galvanizing treatment is not carried out. That is, the coarse bottom dross generation step (S1) is carried out, for example, during a period (stop period) in which the hot-dip galvanizing line equipment is stopped without the steel plate passing through the hot-dip galvanizing equipment.

In the coarse bottom dross generation step (S1), the free Al concentration C _Al and the bath temperature T in the hot-dip galvanizing bath are adjusted so as to satisfy the formula (1), and the coarse size in the hot-dip galvanizing bath is 300 μm or more. Generate bottom dross.
466.15 × C _Al +385.14 ≦ T ≦ 577.24 × C _Al +382.49 (1)
_{Here, the free Al concentration C Al} (mass%) in the hot-dip galvanizing bath 103 _{is substituted for "C Al} " in the formula (1).

_{“466.15 × C Al} +385.14” in the 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. "577.24 x C Al} +382.49" in the formula (1) corresponds to the formula (A). That is, 577.24 × C _Al +382.49 corresponds to the transformation line F ₂₃ in FIG. Thus, equation (1) refers to gamma ₂ grain growth region 22 in FIG.

In the coarse bottom dross generation step (S1), the free Al concentration C _Al and the bath temperature T in the hot dip galvanizing bath 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 dross generated in the hot-dip galvanizing bath 103 is Γ _2- phase dross. As mentioned above, the Γ _2- phase dross has the fastest growth rate among the bottom dross. Furthermore, gamma ₂ grain growth region 22, as compared to the gamma ₂ nucleation region 21, to promote the growth of gamma ₂ phase dross. Therefore, in the coarse bottom dross generation step (S1), coarse bottom dross having a particle size of 300 μm or more can be generated in a relatively short period of time.

In the coarse bottom dross growth step, the period for maintaining the hot-dip galvanizing bath 103 so as to satisfy the formula (1) is not particularly limited. It suffices if a coarse bottom dross having a particle size of 300 μm or more can be generated. It can be confirmed that if the newly constructed hot-dip galvanized bath 103 having no bottom _{dross is maintained for at least 30 days under the condition satisfying the formula (1), Γ 2-} phase dross having a particle size of 300 μm or more is generated. rice field. Therefore, in the coarse bottom dross growth step, it is preferable to maintain the hot dip galvanizing bath 103 for at least 30 days so as to satisfy the formula (1). More preferably, in the coarse bottom dross growth step, the hot dip galvanizing bath 103 is maintained for at least 60 days, more preferably at least 90 days so as to satisfy the formula (1).

In this specification, the particle size of bottom dross is defined as follows. With reference to FIG. 7, in each bottom dross 100, the largest line segment LS among the line segment LS connecting any two points of the interface (that is, the outer circumference of the dross) 150 between the bottom dross 100 and the matrix 200 is “particle size”. Is defined. The particle size can be determined by using image processing on the photographic image in the observation field. In this specification, dross having a maximum diameter of less than 20 μm is excluded because it has almost no effect on dross defects.

[Hot-dip galvanizing process (S2)]
In the hot-dip galvanizing treatment step (S2), the hot-dip galvanizing treatment is performed on the steel sheet by using the hot-dip galvanizing bath 103 after the coarse bottom dross generation step (S1). Specifically, the steel plate is passed through the hot-dip galvanizing bath 103 containing the coarse bottom dross. At this time, a hot-dip galvanized layer is formed on the surface of the steel sheet.

In the hot-dip galvanizing treatment step (S2), during the period during which the hot-dip galvanizing treatment is performed (that is, during the operating period), the free Al concentration C _Al and the bath temperature in the hot-dip galvanizing bath so as to satisfy the formula (2). Adjust T.
390.91 × C _Al +414.20 ≦ T ≦ 485.00 (2)
_{Here, the free Al concentration C Al} (mass%) in the hot-dip galvanizing bath 103 _{is substituted for "C Al} " in the formula (2).

_{“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.}

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 generation step (S1) is raised, and the state of the hot-dip galvanizing bath 103 is changed from the Γ ₂ grain growth region 22. δ Move to the _1- phase nucleation region 31. Then, the state of the hot-dip galvanizing bath 103 is maintained in the δ _1- phase nucleation region 31. At this time, coarse dross in the molten zinc plating bath 103, a phase transformation to [delta] _1-phase from gamma ₂ phase. At the time of phase transformation, part or all of the coarse bottom dross is not dissolved. That is, when the process shifts from the coarse bottom dross growth step (S1) to the hot-dip galvanizing process (S2), the coarse bottom dross deposited on the bottom of the hot-dip galvanized pot 101 does not significantly change in particle size and shape. Phase transformation _{from 2} phase 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 to change the state of the hot-dip galvanizing bath 103 from the Γ ₂ grain growth region 22 to the δ ₁ nucleation region 31. Migrate. Then, the state of the hot-dip galvanizing bath 103 is maintained in the δ ₁ nucleation region 31. In this case, the bottom of the molten zinc pot 101, coarse dross which was transformed into [delta] _1-phase from gamma ₂ phase is present. That is, in the hot-dip galvanizing treatment step (S2), the hot-dip galvanizing treatment is carried out using the hot-dip galvanizing bath 103 including the coarse bottom dross.

In the hot-dip galvanizing treatment step (S2), the state of the hot-dip galvanizing bath 103 is the δ ₁ nucleation region 31 during the period during which the hot-dip galvanizing treatment is being carried out (that is, during the operating period). Therefore, in the hot-dip galvanizing process, fine δ _1- phase dross is generated in the hot-dip galvanizing bath 103. However, there is a coarse bottom dross at the bottom of the hot-dip zinc pot 101. Therefore, due to Ostwald ripening, the fine δ _1- phase dross contracts or disappears, and a coarse bottom dross grows. That is, in the hot-dip galvanizing treatment step (S2), the formation and growth of _{fine bottom dross (fine δ 1-phase dross) is suppressed by utilizing the Ostwald growth of coarse bottom dross.} In this case, the fine δ _1- phase dross becomes smaller and the coarse bottom dross becomes larger. As a result, the formation of medium-sized bottom _{dross (δ 1-} phase dross) having a particle size of 100 to 300 μm can be suppressed. In the hot-dip galvanizing treatment step (S2), the coarse bottom dross further grows. However, since the mass of the coarse bottom dross is large, it is difficult to wind up in the hot-dip galvanizing bath 103 with the accompanying flow. Therefore, the possibility that the coarse bottom dross adheres to the steel sheet is extremely low.

As described above, in the hot-dip galvanizing process (S2), even if fine bottom _{dross (δ 1-} phase dross) is generated during the hot-dip galvanizing process, the fine bottom dross is medium-sized by utilizing the Ostwald growth of the coarse bottom dross. Effectively suppresses the growth of bottom dross. Therefore, it is possible to suppress the formation of dross defects in the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet.

In the hot-dip galvanizing treatment step (S2) described above, even if the hot-dip galvanizing treatment is carried out for a long period of time, the formation of medium-sized bottom dross can be effectively suppressed. Therefore, it is not necessary to carry out the bottom dross removing step, and even when the bottom dross removing step is carried out, the frequency of carrying out the bottom dross removing step can be suppressed. That is, the frequency of stopping the hot-dip galvanizing equipment 10 (stop frequency) can be suppressed. Therefore, the production efficiency can be improved.

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

On the other hand, when the hot-dip galvanizing treatment step (S2) is carried out for a long period of time, the coarse bottom dross deposited on the bottom of the hot-dip zinc pot 101 grows excessively, and the amount of the coarse bottom dross deposited becomes excessively large. In this case, the hot-dip zinc pot 101 may be taken out from the hot-dip galvanizing facility 10 and a bottom dross removing step may be performed. In the bottom dross removing step, at least a part or all of the coarse bottom dross in the hot dip galvanizing bath 103 is removed. The bottom dross removal method may be carried out by a well-known method. In the bottom dross removing step, for example, a bucket suspended from a crane is immersed in a hot-dip zinc pot 101, and the bottom dross is scooped with the bucket. After that, the bucket is pulled up from the hot-dip zinc pot 101, and the bottom dross scooped by the bucket is taken out of the hot-dip zinc pot 101. Further, the bottom dross may be removed by putting Al into the hot-dip zinc pot 101 to convert the bottom dross into a top dross.

A coarse bottom dross generation step (S1) is carried out on the hot dip galvanizing bath 103 from which the bottom dross accumulated on the bottom of the molten zinc pot 101 has been removed. As a result, coarse bottom dross is generated again in the bath-adjusted hot-dip galvanizing bath 103. Then, the bath-adjusted hot-dip galvanizing bath 103 including the coarse bottom dross is re-installed in the hot-dip galvanizing facility 10. Then, the hot-dip galvanizing treatment step (S2) is carried out. In short, the coarse bottom dross generation step (S1) and the hot-dip galvanizing treatment step (S2) may be repeated a plurality of times.

[Method of adjusting _{free Al concentration C Al} and bath temperature T of hot dip galvanizing bath 103]
_{The free Al concentration C Al} and the bath temperature T of the hot-dip galvanizing bath 103 in the above-mentioned coarse bottom dross generation step (S1) and hot-dip galvanizing treatment step (S2) may be adjusted by a well-known 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. Al is added, for example, by immersing the Al ingot in a hot dip galvanizing bath. Al may be added by a method other than dipping the Al ingot in a hot-dip galvanizing bath. When Al is added to the hot-dip galvanizing bath by immersing the Al ingot in the hot-dip galvanizing bath, the Al ingot is placed in the hot-dip galvanizing bath at an immersion rate that can suppress a rapid change in the temperature of the hot-dip galvanizing bath. Immerse. _{The method for adjusting the free Al concentration C Al} in the hot-dip galvanizing bath is not limited to the above method. A well-known method may be used for adjusting the free Al concentration C _{Al in the hot-dip galvanizing bath.}

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

When shifting from the coarse boromudros generation step (S1) to the hot-dip galvanizing treatment step (S2), the hot-dip galvanizing bath 103 satisfying the formula (1) simply by changing the bath temperature of the hot-dip galvanizing bath 103. It is possible to easily shift to the hot-dip galvanizing bath 103 that satisfies (2). Specifically, referring to FIG. 1, in the coarse boromudros 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. It has become. Here, when the transition from the coarse bottom dross generation region (S1) to the hot-dip galvanizing treatment step (S2), if the bath temperature T is raised, the state of the hot-dip galvanizing bath 103 is changed from the Γ ₂ grain growth region to the δ ₁ nuclei. It can be easily moved to the generation area. That is, the hot-dip galvanizing bath 103 satisfying the formula (1) can be easily changed to the hot-dip galvanizing bath 103 satisfying the formula (2) simply by changing the bath temperature T.

In this embodiment, the coarse bottom dross generation step (S1) and the hot-dip galvanizing treatment step (S2) may be alternately and repeatedly performed. When the coarse bottom dross generation step (S1) is carried out after the hot-dip galvanizing treatment step (S2), the bath temperature T of the hot-dip galvanizing bath 103 after the hot-dip galvanizing treatment step (S1) is lowered in the coarse bottom dross step (S1). Then, the hot-dip galvanizing bath 103 satisfying the formula (2) can be changed to the hot-dip galvanizing bath 103 satisfying the formula (1).

In short, in the method for producing a hot-dip galvanized steel sheet of the present embodiment, the hot-dip galvanized bath 103 can be easily brought into a state of satisfying the formula (1) or a state of satisfying the formula (2) simply by changing the bath temperature T. You can switch. Therefore, in the present embodiment, by raising or lowering the bath temperature T, it is possible to switch between the coarse bottom dross generation step (S1) and the hot-dip galvanizing treatment step (S2) extremely easily.

As described above, in the present embodiment, the coarse bottom dross generation step (S1) and the hot-dip galvanizing treatment step (S2) are carried out 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), medium-sized bottom dross, which causes dross defects, is unlikely to be generated. As a result, it is possible to suppress the formation of dross defects in the hot-dip galvanized steel sheet. Further, the frequency of stopping the hot-dip galvanizing equipment 10 (stop frequency) can be suppressed, and the hot-dip galvanizing treatment step (S2) can be carried out for a long period of time.

Further, when the machine is stopped, after adjusting the hot-dip galvanizing bath 103, a coarse bottom dross generation step (S1) is carried out before the hot-dip galvanizing treatment step (S2) is carried out. As a result, when the hot-dip galvanizing process (S2) is performed again after the machine is stopped, the hot-dip galvanizing bath 103 containing the coarse bottom dross in advance can be used in the hot-dip galvanizing process (S2).

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

The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present embodiment includes a step of manufacturing a hot-dip galvanized steel sheet and an alloying treatment step. In the step of manufacturing the hot-dip galvanized steel sheet, the above-mentioned method for manufacturing the hot-dip galvanized steel sheet is carried out. In the alloying treatment step, the hot-dip galvanized steel sheet manufactured in the step of manufacturing the hot-dip galvanized steel sheet is alloyed using the alloying furnace 111 shown in FIG. As the alloying treatment method, it is sufficient to apply a well-known method. An alloyed hot-dip galvanized steel sheet can be manufactured by the above manufacturing process.

The method for producing the hot-dip galvanized steel sheet and the method for producing the alloyed hot-dip galvanized steel sheet of the present embodiment have been described in detail above. In the present embodiment, the hot-dip galvanizing treatment step (S2) is carried out in _{the δ 1} nucleation region 31 by using the hot-dip galvanizing bath 103 containing the coarse bottom dross in advance. Therefore, during the period during which the hot-dip galvanizing treatment is performed (that is, during the operating period), the osteowald growth of the coarse bottom dross in the hot-dip galvanizing bath 103 can be effectively suppressed to effectively suppress the formation and growth of the fine bottom dross. As a result, the generation of medium-sized bottom dross, which causes dross defects, can be suppressed during the period during which the hot-dip galvanizing treatment step is being carried out, and the dross defects of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet can be suppressed.

Further, usually, in the case of the bottom dross operation, when the amount of the bottom dross deposited on the bottom of the hot-dip zinc pot 101 increases, the accumulated medium-sized bottom dross is wound up by the accompanying flow, and dross defects are likely to occur. However, in the case of the present embodiment, a coarse bottom dross is deposited on the bottom of the hot-dip zinc pot 101. The coarse bottom dross has a particle size of 300 μm or more, and due to its mass, it is difficult to wind up due to accompanying flow. Therefore, the coarse bottom dross is unlikely to be a cause of dross defects. Therefore, the frequency of performing the step of removing the bottom dross in the hot-dip 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 is improved.

The method for producing a hot-dip galvanized steel sheet of the present embodiment will be specifically described with reference to an example of the present invention and a comparative example. The examples shown below are merely examples of the method for producing a hot-dip galvanized steel sheet of the present embodiment. Therefore, the method for producing a hot-dip galvanized steel sheet of the present embodiment is not limited to the following examples.

[Example 1]
[Test considering the coarse bottom dross generation process]
We prepared a hot-dip galvanizing bath in a laboratory that imitated an actual 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 galvanized bath was _{held in the Γ 2} grain growth region 22 for 10 days. The free Fe concentration in the hot-dip galvanizing bath at this time was 0.026%. After holding for 10 days, the morphology (phase and particle size) of the bottom dross in the hot-dip galvanized bath was investigated by the following method.

A 300 g sample was collected from the region of the hot-dip galvanizing bath at the center of the depth, the center of the width, and the center of the length. The collected sample was rapidly cooled and solidified. A measurement sample was taken from the solidified sample. The measurement sample was polished by 0.5 mm from the rapidly cooled surface. Of the surfaces of the measurement sample, the polished surface was used as the observation surface. Arbitrary 5 fields of view on the observation surface were observed with a 200x optical microscope. The area of each visual field was 250 μm × 250 μm. In each field of view, the parent phase (Zn) and the dross could be easily distinguished by the contrast. Therefore, the dross in each field of view was measured.

The dross phase of each visual field was identified by the following method. The chemical composition of each dross was analyzed using EPMA. Furthermore, the crystal structure of each dross was analyzed using TEM. As a result, the chemical composition of the dross in each of the five fields of view was composed of 2% Al, 8% Fe, and 90% Zn in mass%, and the crystal structure was face-centered cubic. Therefore, the dross in the hot-dip galvanizing bath was identified as the _{Γ 2-phase dross.} Therefore, the particle size of the gamma ₂ phase dross each field, identified in the manner described above. As a result, the average particle size of all the gamma ₂ phase dross in five fields of view were 100μm or more. The photographic image of FIG. 8A is an example of an image obtained by the scanning electron microscope of Example 1. Particles described as "Γ2" in the figure, a gamma ₂ phase dross. The higher the brightness in the image, the higher the Al concentration (see Al (%) on the right of FIG. 8).

According to the above test, when the free Al concentration C _Al in the hot-dip galvanized bath was maintained at 0.135% and the bath temperature T was maintained at 455 ° C. for 10 days, the hot-dip galvanized bath was used. _{A Γ 2-} phase dross having a particle size of 100 μm or more was formed in. Incidentally, to maintain a free Al concentration C _Al of galvanizing bath was vatting with 0.135%, while maintaining the bath temperature T at 455 ° C., a result of holding for 90 days, at five field gamma ₂ phase The particle size of the dross was 300 μm or more.

[Example 2]
[Test assuming hot-dip galvanizing process]
Next, a hot-dip galvanizing bath in the laboratory was prepared in the same manner as in Example 1. _{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 galvanized bath was _{held in the δ 1} nucleation region 31 for 10 days. After holding for 10 days, the morphology (phase and particle size) of the bottom dross in the hot-dip galvanized bath was investigated by the following method. The morphology (phase and particle size) of the bottom dross in the hot-dip galvanizing bath was examined by the same method as in Example 1. The photographic image of FIG. 8B is an example of an image obtained by the scanning electron microscope of Example 2. Indicated by an arrow in FIG. "Δ1" is a [delta] ₁ phase dross.

As shown in FIG. 8, the chemical composition of the dross in each of the five fields of view is composed of 1% or less of Al, 9% or more of Fe, and 90% or more of Zn in mass%, and the crystal structure is a plane. It was a face-centered cubic crystal. Therefore, the dross in the hot-dip galvanizing bath of Example 2 was recognized as _{δ 1-phase dross.} The particle size of the δ _1- phase dross was measured by the method described above. As a result, the five particle sizes of all the [delta] ₁ phase dross in the field of view is much lower than 100 [mu] m. _{No Γ 2-} phase dross with a particle size of 100 μm or more was confirmed in all five fields of view.

The above test results of Example 1 and Example 2 showed good agreement with the dross phase expected from the metastable phase diagram of FIG. Therefore, it was found that the particle size of the bottom dross can be adjusted by appropriately adjusting _{the free Al concentration C Al} (mass%) of the hot-dip galvanizing bath and the bath temperature T (° C.).

[Example 3]
Based on the above-mentioned Examples 1 and 2, an alloyed hot-dip galvanized steel sheet was manufactured by the following method using the continuous hot-dip galvanized equipment of the actual machine.

In each test number, during the stoppage period, the free Al concentration C _Al (mass%) of the hot-dip galvanizing bath and the bath temperature T (° C.) are shown in the "Al concentration C _Al " column of the "Stop" column in Table 1. And kept as shown in the "bath temperature T" column. The retention period was 30 days.

After the shutdown period had elapsed, a hot-dip galvanizing process was carried out. During the hot-dip galvanizing process, the free Al concentration C _Al (mass%) of the hot-dip galvanizing bath and the bath temperature T (° C.) were maintained as shown in the “Operating” column of Table 1. The retention period was 5 days. The amount of steel sheet passed through during the holding period was the same under each test condition. A well-known alloying treatment was carried out on the steel sheet after the hot-dip galvanizing treatment. The type of steel sheet used in each test number was the same. The conditions for alloying treatment were the same for each test number. Through the above steps, alloyed hot-dip galvanized steel sheets of each test number were manufactured.

The free Al concentration C _Al of each test number was measured over time by the above method to adjust _{the free Al concentration C Al of the hot-dip galvanizing bath.}

The surface of the alloyed hot-dip galvanized steel sheet subjected to the hot-dip galvanized treatment during the final 2 hours of the holding period was visually observed, and the dross defect was evaluated by the following evaluation index.

Specifically, a sample was taken from the center position of an arbitrary width on the surface of the alloyed hot-dip galvanized steel sheet of the alloyed hot-dip galvanized steel sheet. Of the surface of the alloyed hot-dip galvanized layer of the sample, a rectangular region of 1 m × 1 m was defined as one visual field, and any 10 visual fields were set as measurement targets. In each visual field, dross having a particle size of 100 μm or more was visually observed. When dross having a particle size of 100 μm or more adhered to the alloyed hot-dip galvanized layer, it was recognized as a dross defect. The total number of dross defects identified in 10 fields of view was counted. Based on the total number of dross defects and the total area of 10 fields of view (10 m ² ), the number of dross defects per unit area (pieces / 10 m ² ) was determined. The dross, which is difficult to visually determine whether or not the particle size is 100 μm or more, was determined using a 100x optical microscope.

The criteria for dross defect evaluation were as follows.
Evaluation A: The number of dross defects per unit area was 0 to 1/10 m ² .
Evaluation B: The number of dross defects per unit area was 1 to 10/10 m ² .
Evaluation C: The number of dross defects per unit area was 11 pieces / 10 m ² or more.

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

In the "F ₂₁₂₂ " column in _{Table 1, the F 2122} value of the corresponding test number is shown. The "F _{23 " column indicates the F 23} value of the corresponding test number. The "F _{3132 " column shows the F 3132} value of the corresponding test number. In the "Region" column in the "Stopped" column in Table 1, the state of the hot-dip galvanized bath of each test number during the stopped period is shown. For example, in the case of test number 1, it indicates that the state of the hot-dip galvanizing bath during the stoppage period is the Γ ₂ grain growth region. Similarly, in the "Region" column in the "Operating" column in Table 1, the state of the hot-dip galvanizing bath of each test number during the operating period is shown. For example, in the case of test number 1, it indicates that the state of the hot-dip galvanizing bath during the operating period is the δ ₁ grain growth region.

In test numbers 3 to 6, 13 to 16, 24 to 26, and 33 to 35 with reference to Table 1, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath have the formula (1) during the stoppage period. Satisfied. In other words, the state of the hot-dip galvanizing bath was the Γ ₂ grain growth region. Further, during the operating period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath satisfied the formula (2). That is, the state of the hot-dip galvanizing bath was the δ ₁ nucleation region. Therefore, no dross defects were observed in the produced alloyed hot-dip galvanized steel sheet, and the dross defects could be effectively suppressed (evaluation A).

On the other hand, in Test Nos. 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 galvanizing bath instead of [delta] ₁ nucleation region, was [delta] ₁ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip 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 the formula (1), and the state of the hot-dip galvanizing bath was the δ ₁ grain growth region. Further, during the operating 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 the δ ₁ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 10, during the operating 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 the Γ ₂ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 11, during the operating 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 the Γ ₂ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 12, during the operating 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 the δ ₁ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 17, during the operating 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 the δ ₁ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In test numbers 18 and 19, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (1) _{during the shutdown period, and the state of the hot-dip galvanizing bath was the δ 1} nucleation region. .. Therefore, dross defects were confirmed in the manufactured alloyed hot-dip galvanized steel sheet, although the number was small (evaluation B).

In test numbers 20 to 22, 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, and the state of the hot-dip galvanizing bath was the Γ 2} grain growth region. .. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 23, during the operating 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 the δ ₁ grain growth region. Therefore, dross defects were confirmed in the manufactured alloyed hot-dip galvanized steel sheet, although the number was small (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 the formula (1), and the state of the hot-dip galvanizing bath was the δ ₁ grain growth region. Further, during the operating 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 the δ ₁ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 28, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (1) _{during the shutdown period, and the state of the hot-dip galvanizing bath was the δ 1} nucleation region. Therefore, dross defects were confirmed in the manufactured alloyed hot-dip galvanized steel sheet, although the number was small (evaluation B).

In test numbers 30 and 31, 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, and the state of the hot-dip galvanizing bath was the Γ 2} grain growth region. .. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation C).

In Test No. 32, during the operating 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 the δ ₁ grain growth region. Therefore, a dross defect was confirmed in the manufactured alloyed hot-dip galvanized steel sheet (evaluation B).

In test numbers 8, 9 and 29, the bath temperature was constant and was 470 ° C. or higher during both the stoppage period and the operation period. In these test numbers, although the free Al concentration C _Al and bath temperature T of the hot-dip galvanized bath during the shutdown period did not satisfy the formula (1), dross defects were confirmed in the manufactured alloyed hot-dip galvanized steel sheet. Not done (evaluation A). On the other hand, as described above, in test numbers 18, 19 and 28, the bath temperature is constant at 470 ° C. or higher, and the free Al concentration C _Al and the bath temperature T of the hot-dip galvanized bath during the shutdown period are given by the formula (1). However, dross defects were confirmed in the produced alloyed hot-dip galvanized steel sheet (evaluation B). Therefore, in order to suppress dross defects more stably, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath during the shutdown period satisfy the formula (1), and the free Al concentration during the operating period is satisfied. It was considered effective that C _Al and bath temperature T satisfy the formula (2).

[Example 4]
Test No. 4 of Example 3 (Γ _{2 grain growth region during the stoppage period, δ 1} nucleation region during the operation period _{), Test No. 11 (Γ 2} grain growth region during both the stoppage period and the operation period), and The size and number of dross in the hot-dip galvanizing bath of _{test number 18 (δ 1} nucleation region during both the stoppage period and the operation period) were investigated by the following method.

The center position of the length and the center of the width of each hot-dip galvanizing bath after the operation period of test number 4, test number 11, and test number 18 is completed, and the position is 300 mm deep from the liquid surface of the hot-dip galvanizing bath. A sample (liquid phase) was taken from the region. The position at a depth of 300 mm 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, assuming that the depth of the hot-dip galvanizing bath is D (mm).

The collected sample was rapidly cooled using a copper mold and solidified into a rectangular shape. One of the surfaces of the solidified sample was defined as the observation surface. The observation surface was mirror-polished. Of the mirror-polished observation surfaces, an arbitrary 20 mm × 20 mm range was specified as the visual field surface. Using a laser microscope, the particle size and number of bottom dross contained in the visual field surface were measured. Specifically, a 20 mm × 20 mm visual field surface was divided into 100 2 mm × 2 mm fine regions. Each minute region was observed with an optical microscope to generate a photographic image (optical image). In the photographic image of a fine region, the contrast between the matrix (Zn) and the bottom dross is different. Therefore, the photographic image of the fine region was binarized at an appropriate threshold value to clarify the interface 150 between the matrix 200 and the bottom dross 100 as shown in FIG. 7. The bottom dross 100 in the fine region was specified, and the maximum length LS of each of the specified bottom dross 100 was determined by image processing. The determined maximum length was defined as the particle size (μm) of the corresponding bottom dross 100. The bottom dross in all the fine regions was specified, and the particle size of the specified bottom dross was determined. Then, the bottom dross in all the fine regions was classified according to a predetermined particle size range. Then, the number of bottom dross for each class was calculated. The number of bottom dross for each class was made into a histogram.

The above-mentioned sampling position was sufficiently above the coarse bottom dross deposited on the bottom of the hot-dip zinc pot 101. Therefore, the collected sample did not contain the coarse bottom dross deposited on the bottom of the hot-dip zinc pot 101.

The histogram shown in FIG. 9 was created based on the measured particle size and the number of bottom dross.

[Evaluation results]
With reference to FIG. 9, in test number 4 (a Γ _{2 grain growth region during the shutdown period and a δ 1} nucleation region during the operating period), which is an example of the present invention, compared with other test numbers 11 and 18, The total number of bottom dross in the field of view was the smallest. In test number 4, it is considered that the following mechanism worked. Coarse bottom dross was generated by carrying out the coarse bottom dross generation step during the stoppage period. Then, as a result of performing the hot-dip galvanizing treatment during the operation period using the hot-dip galvanizing bath containing the coarse bottom dross, the formation and growth of fine δ1 phase dross were suppressed by the Ostwald ripening. Therefore, the number of bottom dross having a particle size of less than 100 to 300 μm was the smallest, and the total number of bottom dross was also the smallest. As a result, it is probable that no dross defect was confirmed.

On the other hand, in Test No. 11 ( _{Gamma 2} grain growth region during both the stoppage period and the operation period), the bottom dross having a particle size of less than 100 to 300 μm was the largest as compared with Test No. 4 and Test No. 18. Since there were many bottom dross having a particle size of less than 100 to 300 μm, it was expected that there were many dross defects in Test No. 11.

_{In test number 18 (δ 1} nucleation region during both the stoppage period and the operation period), the bottom dross having a particle size of less than 100 to 300 μm was larger than that of test number 4. In particular, the number of bottom dross having a particle size of less than 100 to 150 μm was large. Therefore, it could be expected that there were more dross defects as compared with Test No. 4. In test number 18, since there was no coarse bottom _{dross, the formation and growth of fine δ 1-} phase dross due to Ostwald growth of the coarse bottom dross could not be sufficiently suppressed during the operation period, and as a result, the particle size was less than 100 to 150 μm. It is considered that the number of bottom dross in the above has increased.

[Example 5]
Based on the above test results, the coarse bottom dross generation step is carried out for 30 to 40 days during the shutdown period of the continuous hot dip galvanizing facility, and then the hot dip galvanizing treatment step is carried out using a hot dip galvanizing bath containing the coarse bottom dross. The process of performing for 30-40 days was repeated over a year. At this time, the bath temperature T is raised or so that the hot-dip galvanizing bath in the coarse bottom dross generation step satisfies the formula (1) and the hot-dip galvanizing bath in the hot-dip galvanizing treatment step satisfies the formula (2). I descended and adjusted. As a result, the stop period of 30 to 40 days and the operation period of 30 to 40 days were repeated for one year, and the free Al concentration CAl was kept constant at 0.130% and the bath temperature was 455 ° C. for both the stop period and the operation period. The frequency of performing the bottom dross removal step could be reduced to 1/3 as compared with the case where the operation was carried out at a constant level.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

10 Hot-dip galvanizing equipment 101 Hot-dip galvanized pot 103 Hot-dip galvanizing bath

Claims (8)

A method for manufacturing hot-dip galvanized steel sheets.
_{A coarse bottom dross generation step of adjusting the free Al concentration C Al} and the bath temperature T in the hot-dip galvanizing bath so as to satisfy the formula (1) to generate a coarse bottom dross having a particle size of 300 μm or more in the hot-dip galvanizing bath. When,
_{The free Al concentration C Al} and the bath temperature T of the hot-dip galvanizing bath after the coarse bottom dross generation step are adjusted so as to satisfy the formula (2), and the free Al concentration C _Al and the bath temperature T are expressed by the formula ( The hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on a steel sheet by performing a hot-dip galvanizing treatment using the hot-dip galvanizing bath satisfying 2) is provided.
A method for manufacturing a hot-dip galvanized steel sheet.
466.15 × C _Al +385.14 ≦ T ≦ 577.24 × C _Al +382.49 (1)
390.91 × C _Al +414.20 ≦ T ≦ 485.00 (2)
_{Here, the free Al concentration C Al} (mass%) in the hot-dip galvanizing bath _{is substituted for "C Al} " in the formulas (1) and (2). The method for producing a hot-dip galvanized steel sheet according to claim 1.
When the stop is stopped, the coarse bottom dross generation step is carried out on the hot dip galvanizing bath after the hot dip galvanizing step.
A method for manufacturing a hot-dip galvanized steel sheet. The method for producing a hot-dip galvanized steel sheet according to claim 1 or 2.
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 obtain the hot-dip galvanizing bath satisfying the formula (2).
A method for manufacturing a hot-dip galvanized steel sheet. The method for producing a hot-dip galvanized steel sheet according to claim 3, further
The coarse bottom dross generation step and the hot-dip galvanizing treatment step are alternately repeated.
When the coarse bottom dross generation step is carried out after the hot-dip galvanizing treatment step, in the coarse bottom dross step, the bath temperature T of the hot-dip galvanizing bath after the hot-dip galvanizing treatment step is lowered to obtain the formula (1). ) Is satisfied with the hot-dip galvanizing bath.
A method for manufacturing a hot-dip galvanized steel sheet. The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 4.
_{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 set to 0.125% by mass or more.
A method for manufacturing a hot-dip galvanized steel sheet. The method for producing a hot-dip galvanized steel sheet according to claim 5.
_{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 set to 0.138% by mass or less.
A method for manufacturing a hot-dip galvanized steel sheet. The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 6.
A bottom dross removing step of removing at least a part of the coarse bottom dross in the hot-dip galvanizing bath is provided before the coarse bottom dross generation step is carried out.
A method for manufacturing a hot-dip galvanized steel sheet. A step of manufacturing the hot-dip galvanized steel sheet by carrying out the method for manufacturing the hot-dip galvanized steel sheet according to any one of claims 1 to 7.
The hot-dip galvanized steel sheet is provided with an alloying treatment step for performing an alloying treatment.
A method for manufacturing alloyed hot-dip galvanized steel sheets.

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