Title of the invention: A hot-dip galvanizing method, a method for manufacturing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and a method for manufacturing a hot-dip galvanized steel sheet using the hot-dip galvanizing method.
Technical field
[0001]
The present invention relates to a hot-dip galvanizing method, a method for producing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and a method for producing a hot-dip galvanized steel sheet using the hot-dip galvanizing method.
Background technology
[0002]
The hot-dip galvanized steel sheet (hereinafter, also referred to as GI) and the alloyed hot-dip galvanized steel sheet (hereinafter, also referred to as GA) are manufactured by the following manufacturing process. First, a steel 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. When the base steel sheet is a hot-rolled steel sheet, for example, a pickled hot-rolled steel sheet is prepared. If necessary, the pickled hot-rolled steel sheet may be subjected to a Ni pre-plating treatment to prepare a hot-rolled steel sheet having a Ni layer formed on its surface. A hot-rolled steel sheet that has been subjected to a treatment other than the above may be prepared. When the base steel sheet is a cold-rolled steel sheet, for example, an annealed cold-rolled steel sheet is prepared. If necessary, the annealed cold-rolled steel sheet may be subjected to a Ni pre-plating treatment to prepare a cold-rolled steel sheet having a Ni layer formed on its surface. Cold-rolled steel sheets that have been subjected to treatments other than those described above may be prepared. 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 to perform a hot-dip galvanizing treatment 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.
[0003]
The details of the hot-dip galvanized treatment during 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 facility 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.
[0004]
In the hot-dip galvanizing process, the steel sheet (base 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 onto the surface of the steel sheet from the gas wiping device to scrape off the excess molten zinc on the steel sheet that is pulled up and moves upward, and the amount of plating adhered to the surface of the steel sheet is adjusted. The hot-dip galvanizing treatment step is carried out by the above method. In the case of manufacturing 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.
[0005]
In the above-mentioned hot-dip galvanizing treatment, Fe is eluted in the hot-dip galvanizing bath from the steel sheet immersed in the hot-dip galvanizing bath. When Fe eluted from the steel plate in the hot-dip galvanizing bath reacts with Al and Zn existing in the hot-dip galvanizing bath, an intermetal compound called dross is generated. 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. Of these dross, in particular, the bottom dross is wound up from the bottom of the hot-dip galvanized pot by the accompanying flow generated by the progress of the steel plate in the hot-dip galvanizing bath during the hot-dip galvanizing process, and the hot-dip zinc is rolled up. Floats in the plating 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 become a dot-like defect 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 and corrosion resistance of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets. Therefore, it is preferable that the occurrence of dross defects can be suppressed.
[0006]
A technique for suppressing the occurrence of dross defects has 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).
[0007]
In Patent Document 1, in the method for manufacturing an alloyed hot-dip galvanized steel sheet, the hot-dip galvanized bath temperature is T (° C.), and the boundary Al concentration defined by Cz = −0.0015 × T + 0.76 is Cz (wt%). In this case, the hot-dip galvanized 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%.
[0008]
In Patent Document 2, in the method for manufacturing an alloyed hot-dip galvanized steel sheet, the Al concentration in the bath is kept within the range of 0.15 ± 0.01 wt%. Specifically, Patent Document 2 describes as follows. When the Al concentration in the bath is 0.15 wt% or more, the dross becomes the Fe—Al phase, and when the Al concentration in the bath is 0.15% or less, the dross becomes the delta 1 phase (δ 1 phase). If the dross repeats the phase transformation between the Fe—Al phase and the δ1 phase, the dross becomes finer. Therefore, Patent Document 2 describes that by keeping the Al concentration in the bath within the range of 0.15 ± 0.01 wt%, the dross can be miniaturized, and as a result, the occurrence of dross defects can be suppressed. There is.
Prior art literature
Patent documents
[0009]
Patent Document 1: Japanese Patent Application Laid-Open No. 11-35096
Patent Document 2: Japanese Patent Application Laid-Open No. 11-35097
Non-patent literature
[0010]
Non-Patent Document 1: Practical Applications of Phase Diagrams in Continuous Galvanizing, Nai-Yong Tang, Journal of Phase Equilibria and Diffusion. 27 No. 5,2006
Outline of the invention
Problems to be solved by the invention
[0011]
There are four types of dross that can occur in hot-dip galvanizing treatment: Fe 2Al 5Zn x (so-called top dross), δ 1 phase, gamma 1 phase (Γ 1 phase), and zeta phase (ζ phase). It has been reported in previous studies. For example, Patent Document 2 proposes to miniaturize the δ 1 phase, which is the main cause of dross defects, by operating so that the Al concentration in the bath is near the boundary between the Fe 2Al 5 phase and the δ 1 phase. is doing.
[0012]
However, even when the operation is performed by the method proposed in Patent Document 1 and Patent Document 2, dross defects still occur on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. May be done.
[0013]
Furthermore, in recent years, there has been an increasing demand for alloying hot-dip galvanizing steels containing a large amount of alloying elements such as high-strength steel. It is known that high-strength steel containing a large amount of alloying elements is difficult to alloy in the alloying treatment after the hot-dip galvanizing treatment. Therefore, a steel sheet made of high-strength steel is sometimes referred to as a hard alloying material. There is a demand for a hot-dip galvanizing method that facilitates alloying even for difficult-to-alloy materials. Further, in the case of producing an alloyed hot-dip galvanized steel sheet even if it is not a difficult-to-alloy material, a hot-dip galvanized treatment method in which the alloying treatment is promoted is preferable.
[0014]
An object of the present disclosure is a hot-dip galvanizing treatment method capable of suppressing the occurrence of dross defects and promoting alloying, and a method for manufacturing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing treatment method. It is an object of the present invention to provide a method for manufacturing a hot-dip galvanized steel sheet using the hot-dip galvanized steel sheet.
Means to solve problems
[0015]
The hot-dip galvanizing method according to the present disclosure is a hot-dip galvanizing method used for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet.
A sample collection process in which a sample is collected from a hot-dip galvanizing bath containing Al, and
Using the collected sample, the ζ-phase dross amount determination step for determining the ζ-phase dross amount in the hot-dip galvanizing bath, and
It is equipped with an operating condition adjustment process that adjusts the operating conditions of the hot-dip galvanizing process based on the obtained ζ-phase dross amount.
[0016]
The method for manufacturing alloyed hot-dip galvanized steel sheets according to this disclosure is
The hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on the surface of the steel sheet by implementing the above-mentioned hot-dip galvanizing treatment method on the steel sheet,
The steel sheet having a hot-dip galvanized layer formed on its surface is alloyed to produce an alloyed hot-dip galvanized steel sheet.
[0017]
The manufacturing method of the hot-dip galvanized steel sheet according to this disclosure is
A hot-dip galvanizing treatment step is provided in which the above-mentioned hot-dip galvanizing treatment method is carried out on a steel sheet to form a hot-dip galvanizing layer on the surface of the steel sheet.
The invention's effect
[0018]
The hot-dip galvanizing treatment method according to the present disclosure can suppress the occurrence of dross defects, and can promote alloying even when hot-dip galvanizing treatment and alloying treatment are performed on a high-strength steel steel sheet. Further, the method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present disclosure can manufacture an alloyed hot-dip galvanized steel sheet in which the occurrence of dross defects is suppressed, and further, a hot-dip galvanized steel sheet and an alloy are applied to a high-tensile steel steel sheet. Alloying can be promoted even when the chemical conversion treatment is carried out. The method for producing a hot-dip galvanized steel sheet according to the present disclosure can produce a hot-dip galvanized steel sheet in which the occurrence of dross defects is suppressed.
A brief description of the drawing
[0019]
[Fig. 1] Fig. 1 is a functional block diagram showing an overall configuration of a hot-dip galvanized line facility used for manufacturing an alloyed hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet.
FIG. 2 is a side view of the hot dip galvanizing facility in FIG.
FIG. 3 is a side view of a hot-dip galvanizing facility having a configuration different from that of FIG. 2.
FIG. 4 is a side view of a hot-dip galvanizing facility having a configuration different from that of FIGS. 2 and 3.
FIG. 5 is a functional block diagram showing an overall configuration of a hot-dip galvanizing line facility having a configuration different from that of FIG. 1.
FIG. 6 is a flow chart showing a process of the hot dip galvanizing treatment method of the present embodiment.
FIG. 7 is a diagram showing an example of a photographic image in a part of the observation field of view of a sample collected in the sample collection step of the hot dip galvanizing treatment method of the present embodiment.
Embodiment for carrying out the invention
[0020]
[Factors that cause dross defects]
As mentioned above, in the conventional research, it is reported that the following types of dross generated in the hot-dip galvanizing treatment exist.
(1) Fe 2Al 5Zn x
(2) δ 1-phase dross
(3) Γ 1-phase dross
(4) ζ phase dross
[0021]
Fe 2Al 5Zn x is called top dross. Top dross has a lighter specific density than hot-dip galvanized baths. Therefore, the top dross easily floats on the liquid surface of the hot-dip galvanizing bath. The crystal structure of Fe 2Al 5Zn x is orthorhombic, and its chemical composition 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 always be recovered. Therefore, it is known that top dross is less likely to cause dross defects.
[0022]
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 galvanizing 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.
[0023]
The crystal structure of the δ 1-phase dross is hexagonal, and its chemical composition is 1% or less of Al, 9% or more of Fe, and 90% or more of Zn in mass%. The crystal structure of the Γ1 phase dross is a face-centered cubic crystal, and its chemical composition is 20% Fe and about 80% Zn in mass%. The crystal structure of the ζ-phase dross is a monoclinic crystal, and its chemical composition is 1% or less of Al, 6% of Fe, and 94% of Zn in mass%.To.
[0024]
In previous studies, there were many reports in which the main cause of dross defects was δ 1 phase dross. Also 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 inventor also initially considered that δ 1-phase dross was the main cause of dross defects, and conducted investigations and studies. However, even when the generation of δ 1-phase dross was suppressed in the hot-dip galvanized treatment, dross defects may still occur on the surfaces of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet.
[0025]
Therefore, the present inventor considered that the cause of the dross defect was not the δ1 phase dross but another dross. Therefore, the present inventor re-analyzed the chemical composition and crystal structure of the dross defect portion using the alloyed hot-dip galvanized steel sheet in which the dross defect is generated. The present inventor further analyzed the types of dross generated in the hot-dip galvanizing bath. As a result, the present inventor obtained the following findings regarding the dross defect, which are different from the conventional research results.
[0026]
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: electron probe microanalyzer). 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 a face-centered cubic crystal.
[0027]
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 inventor 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.
[0028]
Therefore, the present inventor has identified the dross that causes the dross defect. Of the above-mentioned dross (1) to (4), the chemical composition of Fe 2Al 5Zn x (top dross) is significantly different from the chemical composition of the dross defect portion. For Γ 1-phase dross, the crystal structure is the same face-centered cubic crystal as the dross defect portion, but the chemical composition (20% Fe by mass% and 80% Zn) is larger than the chemical composition of the dross defect portion. different. 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 (single oblique crystal). ) Is also different from the crystal structure of the dross defect part (face-centered cubic crystal).
[0029]
Based on the above examination results, the present inventor considered that the dross defect was not caused by the dross of (1) to (4) described above. Then, the present inventor considered that the dross defect may be caused by other types of dross other than the above (1) to (4).
[0030]
Therefore, the present inventor further analyzed the dross in the hot-dip galvanizing bath. The above-mentioned EPMA and TEM were used for the analysis of dross. As a result, the present inventor has newly found that gamma 2-phase (Γ2-phase) dross exists as the dross generated in the hot-dip galvanizing bath.
[0031]
The chemical composition of the Γ2-phase dross is 2% Al, 8% Fe, and 90% Zn in mass%, which is consistent with the chemical composition of the analyzed dross defect portion described above. Furthermore, the crystal structure of the Γ2 phase dross is a face-centered cubic crystal, which is consistent with the crystal structure of the dross defect portion. Therefore, the present inventor considered that the Γ2-phase dross was the main cause of the dross defect. Since the specific gravity of the Γ 2-phase dross is larger than the specific gravity of the hot-dip galvanizing bath, the Γ 2-phase dross corresponds to the bottom dross that can be deposited on the bottom of the hot-dip galvanized pot.
[0032]
As mentioned above, Fe 2Al 5Zn x (top dross) has a lighter specific gravity than the hot-dip galvanizing bath. Since Fe 2Al 5Zn x (top dross) floats on the liquid surface of the hot-dip galvanizing bath, it can always be recovered. Therefore, Fe 2Al 5Zn x (top dross) is less likely to cause dross defects.
[0033]
The present inventor further investigated the Γ2 phase dross and the other dross of (2) to (4). 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.
[0034]
As a result of further studies by the present inventor, it was found that among the dross of (2) to (4) above and the Γ2-phase dross, the Γ2-phase dross is a hard dross. Furthermore, it was found that the δ 1-phase dross and the ζ-phase dross are softer than the Γ-phase 2-phase dross, and therefore are less likely to cause dross defects. Further, it was found that the ζ-phase dross is the softest dros among the dross of (2) to (4) above, and the ζ-phase dross is the least likely to cause the dross defect.
[0035]
Based on the above study results, the present inventor has determined 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 not δ 1-phase dross. , Γ We conclude that it is a two-phase dross. Further, the present inventor considers 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 a hot-dip galvanizing bath, Γ 1 We obtained the finding that phase dross is almost nonexistent.
[0036]
The present inventor further obtained the following findings. The ζ-phase dross undergoes phase transformation with each other with the dross of other phases. That is, the Γ 2-phase dross and the ζ-phase dross undergo phase transformation with each other. That is, depending on the conditions of the hot-dip galvanizing treatment, the Γ two-phase dross undergoes a phase transformation into a ζ-phase dross, or the ζ-phase dross undergoes a phase transformation into a Γ-phase two-phase dross. Therefore, if the proportion of the bottom dross in the hot-dip galvanizing bath is increased by the ζ-phase dross, it means that the amount of the Γ2-phase dross in the hot-dip galvanizing bath is relatively reduced.
[0037]
Based on the above findings, the present inventor can adjust the operating conditions of the hot-dip galvanizing treatment so as to intentionally increase the ζ-phase dross, which is the softest and less likely to cause dross defects, which has not been noticed in the past. It has been found that the amount of Γ2-phase dross, which is hard and prone to dross defects in the plating bath, can be reduced, and as a result, dross defects can be suppressed. Then, in the hot-dip galvanizing treatment method, it was considered that the above-mentioned operation can be carried out by controlling the amount of ζ-phase dross in the hot-dip galvanizing bath.
[0038]
[About alloying treatment]
The present inventor further examined the case where the alloying treatment is carried out after the hot-dip galvanizing treatment. In the alloying treatment, Fe contained in the steel sheet diffuses into the hot-dip galvanized layer formed on the surface of the steel sheet to form a Fe—Zn alloy. The alloying process is known to be affected by the Al concentration in the hot dip galvanizing bath. When the Al concentration in the hot-dip galvanizing bath is high, a large amount of Al is also contained in the hot-dip galvanizing layer. Al in the hot-dip galvanized layer inhibits Fe in the steel plate from forming a Fe—Zn alloy with Zn in the hot-dip galvanized layer. That is, when the alloying treatment is taken into consideration, it is preferable that the Al concentration in the hot-dip galvanizing bath is low.
[0039]
In addition, high-strength steel contains a large amount of alloying elements such as Si, P and Mn. The alloying element inhibits the diffusion of Fe in the steel sheet into the hot-dip galvanized layer. Therefore, when hot-dip galvanizing treatment and alloying treatment are performed on high-strength steel, it is preferable that the Al concentration in the hot-dip galvanizing bath is particularly low.
[0040]
On the other hand, if the Al concentration in the hot-dip galvanizing bath is low, Fe eluted from the steel plate into the hot-dip galvanizing bath tends to react with Zn in the hot-dip galvanizing bath. Therefore, if the Al concentration in the hot-dip galvanizing bath is low, the amount of bottom dross increases. Conventionally, it has been considered that the δ 1-phase dross contained in the bottom dross causes the dross defect. Therefore, it has been considered that if the Al concentration in the hot-dip galvanizing bath is lowered, dross defects are likely to occur.
[0041]
However, as a result of the study of the present inventor, if the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the ζ-phase dross, dross defects will be generated even when the Al concentration in the hot-dip galvanizing bath is lowered. It turned out that it can be suppressed. As mentioned above, the ζ phase dross is a type of bottom dross. However, since the ζ-phase dross is soft, it is unlikely to cause dross defects. If the Al concentration in the hot-dip galvanizing bath can be reduced, the formation of Fe—Zn alloy is promoted in the alloying treatment. In this case, even if it is a high-strength steel, alloying becomes easy. That is, the present inventor has found that alloying can be promoted while suppressing dross defects by adjusting the operating conditions of the hot-dip galvanizing treatment so as to increase the ζ-phase dross.
[0042]
As described above, the hot-dip galvanizing treatment method of the present embodiment has been completed based on an idea different from the conventional technical idea, and specifically, it is as follows.
[0043]
The hot-dip galvanizing method of [1] is
A hot-dip galvanized treatment method used in the manufacture of hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets.
A sample collection process in which a sample is collected from a hot-dip galvanizing bath containing Al, and
Using the collected sample, the ζ-phase dross amount determination step for determining the ζ-phase dross amount in the hot-dip galvanizing bath, and
It is provided with an operating condition adjusting step for adjusting the operating conditions of the hot-dip galvanizing treatment based on the obtained ζ-phase dross amount.
[0044]
Here, "adjusting the operating conditions of the hot-dip galvanizing treatment" means adjusting the operating conditions of the hot-dip galvanizing treatment in which the amount of ζ-phase dross in the hot-dip galvanizing bath can be adjusted. Further, "adjusting the operating conditions of the hot-dip galvanizing treatment" includes not only the act of changing the operating conditions of the hot-dip galvanizing treatment but also the act of maintaining the operating conditions as they are.
[0045]
According to the hot-dip galvanizing treatment method having the above-mentioned configuration, the hot-dip galvanizing treatment method is obtained so as to increase the ζ-phase dross amount based on the ζ-phase dross amount in the hot-dip galvanizing bath obtained using the sample. Adjust the operating conditions of. As described above, in the hot-dip galvanizing bath, the amount of ζ-phase dross and the amount of Γ-phase 2-phase dross have a negative correlation. Specifically, if the amount of ζ-phase dross in the hot-dip galvanizing bath is large, it means that the amount of Γ 2-phase dross in the hot-dip galvanizing bath is relatively small. Therefore, the amount of ζ-phase dross in the hot-dip galvanizing bath is obtained, and the operating conditions are adjusted based on the obtained amount of ζ-phase dross to increase the amount of ζ-phase dross. Can be reduced. As a result, the occurrence of dross defects can be suppressed. Further, since the amount of Γ2-phase dross is reduced by increasing the ζ-phase dross, the dross defect can be suppressed even if the Al concentration in the hot-dip galvanizing bath is lowered. If the Al concentration in the hot-dip galvanizing bath can be reduced, alloying is promoted.
[0046]
The hot dip galvanizing treatment method of this embodiment can be suitably applied to high-strength steel. The hot-dip galvanizing treatment method of the present embodiment can promote alloying even for steels other than high-strength steel. Therefore, the hot-dip galvanizing treatment method of the present embodiment can be suitably applied to steels other than high-strength steel. In the present specification, the high-strength steel means a steel having a tensile strength of 340 MPa or more. In the present specification, the steel other than the high-strength steel means a steel having a tensile strength of less than 340 MPa.
[0047]
The hot-dip galvanizing treatment method of [2] is the hot-dip galvanizing treatment method according to [1].
In the ζ phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area is determined as the amount of ζ-phase dross.
[0048]
Here, the predetermined area is particularly limited Not determined. The predetermined area may be, for example, the entire area of ​​the observation field of view in the case of observing the ζ-phase dross in the predetermined observation field of view using a sample, or may be a unit area (cm 2).
[0049]
The hot-dip galvanizing treatment method of [3] is the hot-dip galvanizing treatment method according to [1] or [2].
In the operating condition adjustment process,
Based on the obtained ζ-phase dross amount, at least one of (A) or (B) is carried out to increase the ζ-phase dross amount.
(A) Adjust the bath temperature of the hot-dip galvanizing bath.
(B) Adjust the Al concentration of the hot-dip galvanizing bath.
[0050]
Both (A) and (B) above are effective operating conditions for phase-transforming the dross of another phase into the ζ-phase dross and increasing the generation of the ζ-phase dross. Therefore, by carrying out at least one of (A) or (B), the amount of ζ-phase dross can be increased and the amount of Γ-phase 2-phase dross can be decreased.
[0051]
The hot-dip galvanizing treatment method of [4] is the hot-dip galvanizing treatment method according to any one of [1] to [3].
In the operating condition adjustment process,
When the obtained ζ-phase dross amount is less than the threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the ζ-phase dross amount.
[0052]
In this case, whether or not to change the operating conditions can be easily determined based on the ζ-phase dross amount and the threshold value. For example, when the obtained ζ-phase dross amount is less than the threshold value, the operating conditions can be adjusted so that the ζ-phase dross amount increases. More preferably, when the obtained ζ-phase dross amount is less than the threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted so that the ζ-phase dross amount is equal to or more than the threshold value.
[0053]
The hot-dip galvanizing treatment method of [5] is the hot-dip galvanizing treatment method according to [4].
In the ζ phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area was determined as the amount of ζ-phase dross.
In the operating condition adjustment process,
When the obtained ζ-phase dross amount is less than 5.0 pieces / cm 2 when converted into a unit area (1 cm 2), the operating conditions of the hot-dip galvanizing treatment are adjusted to obtain the ζ-phase dross amount. To increase.
[0054]
In this case, the Γ-phase 2-phase dross is relatively reduced by keeping the ζ-phase dross amount high. As a result, the occurrence of dross defects due to Γ2 phase dross can be more effectively suppressed.
[0055]
The hot-dip galvanizing treatment method of [6] is the hot-dip galvanizing treatment method according to any one of [1] to [5].
In the operating condition adjustment process,
When the Fe concentration in the hot-dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass%), the Fe concentration and Al in the hot-dip galvanizing bath are defined. The concentration is adjusted to satisfy the formulas (1) and (2).
0.100 ≤ Y ≤ 0.139 (1)
Y ≤ 0.2945X + 0.1216 (2)
[0056]
Here, the Fe concentration in the hot-dip galvanizing bath means the Fe concentration (so-called Free-Fe concentration) melted in the hot-dip galvanizing bath. That is, in the present specification, the "Fe concentration in the hot-dip galvanizing bath" is melted in the hot-dip galvanizing bath (that is, the liquid) excluding the Fe content contained in the dross (top dross and bottom dross). Means Fe concentration (in phase). Similarly, the Al concentration in the hot-dip galvanizing bath means the Al concentration (so-called Free-Al concentration) melted in the hot-dip galvanizing bath. That is, in the present specification, the "Al concentration in the hot-dip galvanizing bath" is melted in the hot-dip galvanizing bath (that is, liquid) excluding the Al content contained in the dross (top dross and bottom dross). It means the Al concentration (in the phase).
[0057]
In this case, the amount of ζ phase dross increases, and as a result, the amount of Γ2 phase dross decreases relatively. Therefore, it is possible to more effectively suppress the occurrence of dross defects caused by Γ2 phase dross.
[0058]
The hot-dip galvanizing treatment method of [7] is the hot-dip galvanizing treatment method according to [6].
In the operating condition adjustment process,
When the Fe concentration in the hot-dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass%), the Fe concentration and Al in the hot-dip galvanizing bath are defined. The concentration is adjusted to satisfy the formulas (1) and (3).
0.100 ≤ Y ≤ 0.139 (1)
Y ≤ 0.2945X + 0.1066 (3)
[0059]
In this case, the amount of ζ phase dross is further increased, and as a result, the amount of Γ2 phase dross is further decreased. Therefore, it is possible to more effectively suppress the occurrence of dross defects caused by Γ2 phase dross.
[0060]
The hot-dip galvanizing treatment method of [8] is the hot-dip galvanizing treatment method according to any one of [1] to [7].
In the hot-dip galvanized pot in which the hot-dip galvanizing bath is stored, a sink roll for contacting the steel sheet immersed in the hot-dip galvanizing bath and changing the traveling direction of the steel sheet up and down is arranged. ,
In the sample collection process,
The sample is collected from the depth range from the upper end to the lower end of the sink roll in the hot dip galvanizing bath in the hot dip galvan pot.
[0061]
In this case, take a sample from the same depth range as the sink roll. Therefore, the correlation between the ζ-phase dross amount and the dross defect can be further enhanced.
[0062]
The method for manufacturing the alloyed hot-dip galvanized steel sheet in [9] is
A hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on the surface of the steel sheet by carrying out the hot-dip galvanizing treatment method according to any one of [1] to [8] on the steel sheet.
The steel sheet having the hot-dip galvanized layer formed on the surface thereof is subjected to an alloying treatment to produce the alloyed hot-dip galvanized steel sheet.
[0063]
As the method for manufacturing the alloyed hot-dip galvanized steel sheet of the present embodiment, the above-mentioned hot-dip galvanized treatment method of the present embodiment is applied. Therefore, it is possible to manufacture an alloyed hot-dip galvanized steel sheet in which dross defects are suppressed. Further, even when hot-dip galvanizing treatment and alloying treatment are performed on high-strength steel, alloying can be promoted.
[0064]
The method for manufacturing the hot-dip galvanized steel sheet in [10] is
A hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on the surface of the steel sheet by carrying out the hot-dip galvanizing treatment method according to any one of [1] to [8] on the steel sheet is provided.
[0065]
As the method for manufacturing the hot-dip galvanized steel sheet of the present embodiment, the above-mentioned hot-dip galvanized treatment method of the present embodiment is applied. Therefore, it is possible to manufacture a hot-dip galvanized steel sheet in which dross defects are suppressed.
[0066]
Hereinafter, the hot-dip galvanizing treatment method, the alloying hot-dip galvanized steel sheet manufacturing method, and the hot-dip galvanized steel sheet manufacturing method according to the present embodiment will be described with reference to the drawings. In the present specification and the drawings, the configurations having substantially the same functions are designated by the same reference numerals and the description thereof will not be repeated.
[0067]
[About the configuration of hot-dip galvanizing line equipment]
FIG. 1 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanized line facility used for manufacturing an alloyed hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet. With reference to FIG. 1, the hot-dip galvanizing line facility 1 includes an annealing furnace 20, a hot-dip galvanizing facility 10, and a tempering rolling mill (skin pass mill) 30.
[0068]
The annealing furnace 20 includes one or more heating zones (not shown) and one or more cooling zones 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, and an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet is manufactured. The tempering rolling mill 30 is arranged downstream of the hot dip galvanizing facility 10. In the tempering rolling mill 30, the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet manufactured in the hot-dip galvanized equipment 10 is lightly reduced as necessary to the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. Adjust the surface of the galvanized steel sheet.
[0069]
[About hot-dip galvanizing equipment 10]
FIG. 2 is a side view of the hot-dip galvanizing facility 10 in FIG. With reference to FIG. 2, the hot-dip galvanizing facility 10 includes a hot-dip galvan pot 101, a sink roll 107, a support roll 113, a gas wiping device 109, and an alloying furnace 111.
[0070]
The annealing furnace 20 located upstream of the hot-dip galvanizing facility 10 is shielded from the atmosphere inside and is maintained in a reducing atmosphere. As described above, the annealing furnace 20 heats the steel sheet S that is continuously conveyed in the heating zone. As a result, the surface of the steel sheet S is activated and the mechanical properties of the steel sheet S are adjusted.
[0071]
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 201 is arranged. The downstream end of the annealing furnace 20 is connected to the upstream end of the snout 202. The downstream end of the snout 202 is immersed in the hot dip galvanizing bath 103. The inside of the snout 202 is shielded from the atmospheric atmosphere and is maintained in a reducing atmosphere.
[0072]
The steel plate S whose transport direction has been changed downward by the turndown roll 201 passes through the snout 202 and is continuously immersed in the hot-dip galvanizing bath 103 stored in the hot-dip galvanized 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 facility 10.
[0073]
The support roll 113 is in the hot-dip galvanizing bath 103 and is arranged 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 by sandwiching the steel plate S whose traveling direction is changed upward by the sink roll 107.
[0074]
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 have gas injection nozzles that oppose each other. During the hot-dip galvanizing process, the steel sheet S passes between the pair of gas injection nozzles of the gas wiping device 109. At this time, the pair of gas injection nozzles face the surface of the steel plate S. The gas wiping device 109 sprays gas on both surfaces of the hot-dip galvanized steel sheet S pulled up from the hot-dip galvanized bath 103 to scrape off a part of the hot-dip galvanized sheet adhering to both surfaces of the hot-dip galvanized steel sheet S. Adjust the amount of hot-dip galvanized on the surface.
[0075]
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 tropics maintain 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 11 In No. 1, the above-mentioned alloying treatment is carried out when the alloyed hot-dip galvanized steel sheet is manufactured. On the other hand, when the hot-dip galvanized steel sheet is manufactured, 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 115.
[0076]
When manufacturing 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 115 without passing through the alloying furnace 111.
[0077]
When the hot-dip galvanizing facility 10 is a facility dedicated to the hot-dip galvanized steel sheet, the hot-dip galvanizing facility 10 does not have to be equipped with the alloying furnace 111 as shown in FIG.
[0078]
[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 quenching furnace 20 and the hot-dip galvanizing facility 10 as shown in FIG. The pre-plating equipment 40 may be arranged. The Ni pre-plating equipment 40 includes a Ni plating cell for storing a Ni plating bath. The Ni plating process is carried out by an electroplating method. The hot-dip galvanizing line equipment 1 of FIGS. 1 and 5 includes an annealing furnace 20 and a temper rolling mill 30. However, the hot dip galvanizing line equipment 1 does not have to be provided with the annealing furnace 20. Further, the hot-dip galvanizing line equipment 1 does not have to be equipped with the tempering rolling mill 30. 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 30 may be arranged as needed. Further, the hot-dip galvanizing line facility 1 may be provided with a pickling facility for pickling a steel plate upstream of the hot-dip galvanizing facility 10, or other facilities other than the annealing furnace 20 and the pickling facility. May be provided. The hot-dip galvanizing line equipment 1 may be further provided with equipment other than the tempering and rolling mill 30 downstream of the hot-dip galvanizing equipment 10.
[0079]
[About the mechanism of dross defect generation]
The mechanism by which dross defects occur in the hot-dip galvanizing process during the manufacturing process of the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet using the above-mentioned hot-dip galvanizing line equipment 1 is considered to be as follows.
[0080] [0080]
In the hot-dip galvanizing treatment step, Fe melts into the hot-dip galvanizing bath 103 from the steel plate S immersed in the hot-dip galvanizing bath 103. The melted Fe reacts with Al and / or Zn in the hot-dip galvanizing bath 103 to generate dross. Of the generated dross, the top dross floats on the liquid surface in the hot-dip galvanizing bath 103. On the other hand, of the generated dross, the bottom dross sinks and accumulates at the bottom of the hot-dip zinc pot 101. When the production of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is repeated (that is, as the amount of the steel sheet S passing through the hot-dip galvanized bath 103 increases), the bottom dross is deposited on the bottom of the hot-dip galvanized pot 101.
[0081]
The bottom dross deposited on the bottom of the hot-dip galvanized pot 101 is wound up in the hot-dip galvanizing bath 103 by the accompanying flow of the steel plate S generated near the lower part of the sink roll 107, and floats in the hot-dip galvanizing bath 103. The bottom dross floating in the hot-dip galvanizing bath 103 adheres to the surface of the steel sheet S in the vicinity of the sink roll 107. The portion where the bottom dross adheres to the surface of the steel sheet S may become a dross defect.
[0082]
If a dross defect occurs, a non-uniform portion of the plating will occur on the plated surface, and the appearance quality of the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet will deteriorate. Further, a local battery is likely to be formed on the dross defect portion on the surface of the steel sheet, and the corrosion resistance of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is lowered.
[0083]
As mentioned above, the main cause of dross defects is not the δ 1-phase dross reported in many previous studies, but the Γ 2-phase dross. Therefore, if the amount of Γ2-phase dross in the hot-dip galvanized bath 103 is large, there is a high possibility that dross defects will occur in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet.
[0084]
Furthermore, the ζ-phase dross and the Γ-phase 2-phase dross undergo phase transformation with each other. That is, the ζ-phase dross undergoes a phase transformation into a Γ-phase 2-phase dross, and the Γ-phase 2-phase dross undergoes a phase transformation into a ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the amount of ζ-phase dross and the amount of Γ2-phase dross have a negative correlation, and if the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is large, the hot-dip galvanizing bath 103. It means that the amount of Γ2-phase dross in Γ is relatively small. Furthermore, the ζ phase dross is the softest compared to the dross of other phases and is less likely to cause dross defects. Therefore, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is obtained, and the operating conditions are adjusted based on the obtained amount of ζ-phase dross to increase the amount of ζ-phase dross, thereby increasing Γ 2 in the hot-dip galvanizing bath 103. The amount of phase dross can be reduced. As a result, the occurrence of dross defects can be suppressed.
[0085]
Therefore, in the hot-dip galvanizing treatment method of the present embodiment, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is obtained. Then, the operating conditions of the hot-dip galvanizing process are adjusted based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103. Preferably, the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the amount of ζ-phase dross based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103. As a result, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be increased, and the amount of Γ-phase 2-phase dross can be kept relatively low. As a result, it is possible to suppress the occurrence of dross defects in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. Preferably, based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the ζ-phase dross, and the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is adjusted. Keep above a certain amount (threshold).
[0086]
The hot-dip galvanized treatment method of the present embodiment can be applied to a method for manufacturing an alloyed hot-dip galvanized steel sheet (GA) and also to a method for manufacturing a hot-dip galvanized steel sheet (GI). Hereinafter, the hot dip galvanizing treatment method of the present embodiment will be described in detail.
[0087]
[About the hot-dip galvanizing treatment method of this embodiment]
[About the hot-dip galvanizing equipment to be used]
In the hot-dip galvanizing treatment method of this 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. 1 and 5. However, 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. 1 and 5, or the equipment shown in FIGS. 1 and 5. Further other configurations may be added. Further, a well-known hot-dip galvanizing line facility 1 having a configuration different from that of FIGS. 1 and 5 may be used.
[0088]
[About steel sheets used for hot-dip galvanizing]
The steel type and size (plate thickness, plate width, etc.) of the steel plate (base steel plate) used for the hot-dip galvanizing treatment 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 the automobile may be used as the steel sheet (base steel sheet) used for the hot-dip galvanizing treatment.
[0089]
As described above, in the hot-dip galvanizing treatment of the present embodiment, dross defects can be suppressed even if the Al concentration in the hot-dip galvanizing bath 103 is reduced. Therefore, alloying can be promoted by reducing the Al concentration in the hot-dip galvanizing bath 103. The steel sheet used for the hot-dip galvanizing treatment of the present embodiment may be a steel sheet made of high-strength steel containing a large amount of alloying elements such as Si and Mn. The steel sheet used for the hot-dip galvanizing treatment of the present embodiment may be a steel sheet made of steel other than high-strength steel.
[0090]
The steel sheet (base steel sheet) used for the hot-dip galvanizing treatment of the present embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet. As the base steel plate, for example, the following steel plate is used.
(A) Pickled hot-rolled steel sheet
(B) Hot-rolled steel sheet that has been pickled and then subjected to Ni pre-plating treatment to form a Ni layer on the surface.
(C) Annealed cold-rolled steel sheet
(D) Cold-rolled steel sheet that has been annealed and then subjected to Ni pre-plating to form a Ni layer on the surface.
The above (a) to (d) are examples of the steel sheet used for the hot-dip galvanizing treatment of the present embodiment. The steel sheet used for the hot-dip galvanizing treatment of the present embodiment is not limited to the above (a) to (d). A hot-rolled steel sheet or a cold-rolled steel sheet that has been subjected to a treatment other than the above (a) to (d) may be used as a hot-dip galvanized steel sheet.
[0091]
[About hot-dip galvanizing bath]
The main component of the hot-dip galvanizing bath 103 is Zn. The hot dip galvanizing bath 103 further contains Al in addition to Zn. That is, the hot-dip galvanizing bath 103 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. If the hot-dip galvanizing bath 103 contains a specific concentration of Al, it is possible to suppress an excessive reaction between Fe and Zn in the bath, and the steel plate immersed in the hot-dip galvanizing bath 103 and Zn are not compatible. The progress of a uniform alloy reaction can be suppressed.
[0092]
The preferable Al concentration (more specifically, Free-Al concentration) in the hot-dip galvanizing bath 103 is 0.100 to 0.159% by mass. Here, the Al concentration in the hot-dip galvanizing bath 103 means the Al concentration (mass%) dissolved in the hot-dip galvanizing bath 103, and means the so-called Free-Al concentration. When the Al concentration in the hot-dip galvanizing bath is in the range of 0.100 to 0.159% in mass%, it is possible to suppress the occurrence of other pattern defects different from the dross defects, and further, the alloyed hot-dip zinc. It is possible to suppress the generation of unalloy in the alloying process during the manufacturing process of the plated steel sheet.
[0093]
As described above, the hot-dip galvanizing bath 103 according to the present embodiment is a plating bath containing Zn as a main component and further containing Al. The hot-dip galvanizing bath 103 may further contain 0.020 to 0.100% by mass of Fe eluted from the equipment in the bath or the steel plate. That is, the Fe concentration (mass%) dissolved in the hot-dip galvanizing bath 103 is, for example, 0.020 to 0.100% by mass. However, the Fe concentration dissolved in the hot-dip galvanizing bath 103 is not limited to the above numerical range.
[0094]
[Hot-dip galvanizing method]
The hot-dip galvanizing treatment method of the present embodiment uses a hot-dip galvanizing bath 103 containing Al. FIG. 6 is a flow chart showing a process of the hot dip galvanizing treatment method of the present embodiment. With reference to FIG. 6, the hot-dip galvanizing treatment method of the present embodiment includes a sample sampling step (S1), a ζ phase dross amount determination step (S2), and an operating condition adjusting step (S3). Hereinafter, each step will be described in detail.
[0095]
[Sample collection process (S1)]
In the sample collection step (S1), a part of the plating solution is collected as a sample from the hot-dip galvanizing bath 103. In the sample collection step (S1), a sample is collected over time. "Taking a sample over time" means taking a sample every time a specific time elapses. The specific time (the period from one sample to the next) may or may not be constant. For example, a sample may be taken hourly. Further, the next sample may be collected 1 hour after the sample is collected, and the next sample may be collected 30 minutes later. identification The time is not particularly limited.
[0096]
The amount of sample collected from the hot-dip galvanizing bath 103 is not particularly limited. In the ζ-phase dross amount determination step (S2) of the next step, the sample collection amount is not particularly limited as long as the amount of the ζ-phase dross in the hot-dip galvanizing bath 103 can be determined. The sample collection amount is, for example, 100 to 400 g. The collected sample may be brought into contact with a metal at room temperature having a high thermal conductivity, and the sample may be rapidly cooled to room temperature to solidify. A metal at room temperature having high thermal conductivity is, for example, copper.
[0097]
The sampling position in the hot-dip galvanizing bath 103 is not particularly limited. For example, referring to FIGS. 2 to 4, when the hot-dip galvanizing bath 103 is divided into three equal parts D1 to D3 in the depth direction D, a sample is taken in the uppermost region D1 in the hot-dip galvanizing bath 103. A sample may be collected in the central region D2, or a sample may be collected in the lowermost region D3. The amount of ζ-phase dross in the samples collected in each region D1 to D3 is different. However, it is possible to determine to some extent whether or not the obtained ζ-phase dross amount is large depending on the sampling position. Therefore, the sampling position is not particularly limited. As shown in FIGS. 2 to 4, in the hot-dip zinc plating bath 103, the direction parallel to the plate width direction of the steel plate S is defined as the width direction W, and the depth direction of the hot-dip zinc plating bath 103 is defined as the depth direction D. It is defined, and the direction perpendicular to the width direction W and the depth direction D is defined as the length direction L. In this case, it is preferable to sample over time from within a specific region partitioned by a specific width range in the width direction W, a specific depth range in the depth direction D, and a specific length range in the length direction L. To collect. In short, a sample is taken over time from the same position (within a specific region) in the hot-dip galvanizing bath 103.
[0098]
Preferably, take a sample from the area near the sink roll 107 as much as possible. Specifically, as shown in FIGS. 2 to 4, a sample is taken from within a specific depth range D107 from the upper end to the lower end of the sink roll 107 in the depth direction D of the hot-dip galvanizing bath 103. .. That is, the specific depth range is the depth range D107 from the upper end to the lower end of the sink roll 107. The Γ 2-phase dross is likely to adhere to the surface of the steel sheet S near the sink roll 107. Therefore, the amount of ζ-phase dross in the vicinity of the sink roll 107 is the most effective index for suppressing the dross defect. Therefore, it is preferable to take a sample from the depth range D107. In this case, since the ζ-phase dross amount is obtained based on the sample taken from the range where the steel sheet S is most likely to adhere to the surface, the correlation between the ζ-phase dross amount and the dross defect can be further enhanced. Also in the width direction W and the length direction L, it is preferable to collect a sample from a region near the sink roll as much as possible. As described above, the sample is collected over time from the same region in the hot-dip galvanizing bath 103.
[0099]
[Ζ phase dross amount determination step (S2)]
In the ζ-phase dross amount determination step (S2), the ζ-phase dross amount in the hot-dip galvanizing bath 103 is obtained using the collected sample. The method for obtaining the ζ-phase dross amount using a sample is not particularly limited, and various methods can be considered.
[0100]
For example, a test piece for ζ phase dross observation is prepared from the sample collected in the sample collection step (S1). As an example of the ζ-phase dross observation test piece, a rectangular parallelepiped (small plate shape) having a surface (inspection surface) capable of securing an observation field of view of 15 mm × 15 mm and a thickness of 0.5 mm is used. Using an optical microscope or a scanning electron microscope (SEM) with a predetermined magnification, full-field observation is performed in the above observation field (15 mm × 15 mm), and dross in the entire field is specified. The contrast in the visual field can identify the dross, and the contrast can distinguish between the top dross and the bottom dross.
[0101]
FIG. 7 is an example of a photographic image in a part of the observation field of view of the sample collected in the sample collection step (S1). With reference to FIG. 7, hot-dip galvanized matrix 200, top dross 100T, and bottom dross 100B are observed in the photographic image. The top dross 100T has a lower brightness (darker) than the mother phase 200 and the bottom dross 100B. On the other hand, the bottom dross 100B has a lower brightness (darker) than the mother phase 200 and a higher brightness (brighter) than the top dross 100T. As described above, the top dross and the bottom dross can be distinguished based on the contrast.
[0102]
Of the dross identified in the above observation field of view (15 mm × 15 mm), composition analysis using EPMA is performed for each bottom dross to identify the ζ phase dross. Further, crystal structure analysis using TEM may be carried out for each bottom dross to identify the ζ phase dross in the observation field of view. In addition, without distinguishing between top dross and bottom dross by contrast, composition analysis using EPMA and / or crystal structure analysis using TEM was performed for each dros, and the type of each dros in the visual field (top). Dross, Γ 2-phase dross, δ 1-phase dross, and ζ-phase dross) may be specified.
[0103]
Based on the specified ζ-phase dross, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is obtained. The amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be determined by various indexes. For example, the number of ζ-phase dross per predetermined area may be used as the ζ-phase dross amount. Here, the predetermined area is not particularly limited, and may be, for example, the entire area of ​​the observation field of view or a unit area (mm 2). For example, when the observation field of view is 15 mm × 15 mm, the number of ζ-phase dross (pieces / 225 mm 2) in the observation field of view (15 mm × 15 mm = 225 mm 2) may be used as the ζ-phase dross amount. In this case, the number of ζ-phase dross in the observation field is determined by the following method. First, the equivalent circle diameter (μm) of the specified ζ-phase dross is determined. The diameter when the area of ​​each ζ-phase dross in the above-mentioned observation field is converted into a circle is defined as the equivalent circle diameter (μm). Using the photographic image of the observation field, a circle-equivalent diameter (μm) of the specified ζ-phase dross is obtained by well-known image processing. In the field of view, the number of ζ-phase dross having a circle-equivalent diameter of 10 μm or more is defined as the number of ζ-phase dross (pieces / 225 mm 2). As described above, the number of ζ-phase dross having a diameter equivalent to a circle of 10 μm or more in the observation field may be defined as the amount of ζ-phase dross. The observation field of view is not limited to the above region (15 mm × 15 mm = 225 mm 2). Further, the upper limit of the equivalent circle diameter of the ζ-phase dross is not particularly limited. The upper limit of the equivalent circle diameter of the ζ-phase dross is, for example, 300 μm.
[0104]
Alternatively, another index may be the amount of ζ phase dross in the hot-dip galvanizing solution. For example, in the above-mentioned observation field of view, the area of ​​each bottom dross (each Γ 2-phase dross, each δ 1-phase dross, and each ζ-phase dross) and the area of ​​each ζ-phase dross are obtained. Then, the ratio of the total area of ​​the ζ-phase dross to the total area of ​​the bottom dross may be used as the ζ-phase dross amount. Further, the ratio of the total area of ​​the ζ-phase dross to the observation field area may be used as the ζ-phase dross amount. Further, the total area (μm 2) of the ζ-phase dross in the above-mentioned visual field may be used as the ζ-phase dross amount. In addition, X-ray diffraction measurement is performed on the surface to be inspected of the above-mentioned sample to measure the peak intensity of each bottom dross (Γ 2-phase dross, δ 1-phase dross, and ζ-phase dross). Then, the ratio of the peak intensity of the ζ-phase dross to the sum of the peak intensities of each bottom dross (that is, the peak intensity of the Γ2-phase dross, the peak intensity of the δ1-phase dross, and the sum of the peak intensities of the ζ-phase dross) is calculated. The amount of ζ-phase dross may be used. In X-ray diffraction measurement, it is not easy to clearly distinguish between Γ 2-phase dross and Γ 1-phase dross. However, as described above, it is considered that the Γ1 phase dross is almost nonexistent. Therefore, all the peak intensities obtained at the diffraction angle 2θ = 43 to 44 ° are regarded as the peak intensities of the Γ2 phase dross. For example, a Co dry ball is used as a target for X-ray diffraction measurement. The ζ-phase dross amount may be obtained by a method other than the above.
[0105]
By the above method, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is determined using the sample collected in the sample collection step (S1). The ζ-phase dross amount determination step (S2) is preferably performed every time a sample is taken in the sample taking step (S1). By collecting a sample over time and determining the ζ-phase dross amount each time the sample is taken, it is possible to grasp the change over time in the ζ-phase dross amount in the hot-dip galvanizing bath 103. Therefore, the amount of ζ phase dross may be determined over time based on the sample taken over time.
[0106]
[Operating condition adjustment process (S3)]
After determining the ζ-phase dross amount in the hot-dip galvanizing bath 103 in the ζ-phase dross amount determination step (S2), the operating condition adjustment step (S3) is carried out.
[0107]
In the operating condition adjustment step (S3), the operating conditions of the hot-dip galvanizing treatment are adjusted based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103. Specifically, when the obtained ζ-phase dross amount is small, the operating conditions are adjusted (changed) so as to increase the ζ-phase dross amount in the hot-dip galvanizing bath 103. If the obtained ζ-phase dross amount is an appropriate amount, the operating conditions may be maintained as they are. The method for adjusting the operating conditions is not particularly limited as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted. Specifically, as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted, the method for adjusting the operating conditions is not particularly limited.
[0108]
Preferably, at least one of the following (A) or (B) is carried out as a method for adjusting the operating conditions.
(A) Adjust the bath temperature of the hot-dip galvanizing bath 103.
(B) Adjust the Al concentration of the hot-dip galvanizing bath 103.
[0109]
Regarding (A) above, if the temperature of the hot-dip galvanizing bath 103 is raised, the possibility that the Γ 2-phase dross undergoes a phase transformation into the ζ-phase dross increases. Therefore, if the temperature of the hot-dip galvanizing bath 103 is increased, the Γ two-phase dross in the hot-dip galvanizing bath 103 decreases, and instead, the ζ-phase dross increases. As mentioned above, the ζ phase dross is soft. Therefore, the ζ-phase dross is unlikely to form a dross defect. Therefore, when the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is excessively small, the bath temperature of the hot-dip galvanizing bath 103 may be increased. In this case, the hard Γ two-phase dross undergoes a phase transformation into a soft ζ-phase dross. As a result, the soft ζ-phase dross increases and the hard Γ-phase 2-phase dross decreases. Therefore, the occurrence of dross defects is suppressed. Increasing the bath temperature increases the energy intensity. Therefore, when the amount of ζ-phase dross is sufficiently large, it is not necessary to raise the bath temperature excessively. As described above, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted by adjusting the bath temperature of the hot-dip galvanizing bath 103. Specifically, by increasing the bath temperature of the hot-dip galvanizing bath 103, the amount of ζ-phase dross can be increased, and as a result, the amount of Γ-phase 2-phase dross in the hot-dip galvanizing bath 103 can be reduced.
[0110]
Regarding (B) above, if the Al concentration in the hot-dip galvanizing bath 103 is lowered, the possibility that the Γ 2-phase dross undergoes a phase transformation into the ζ-phase dross increases. Therefore, when the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is excessively small, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted by adjusting the Al concentration in the hot-dip galvanizing bath 103. Specifically, by reducing the Al content of the hot-dip galvanizing bath 103, the amount of ζ-phase dross can be increased, and as a result, the Γ-phase 2-phase dross in the hot-dip galvanizing bath 103 can be reduced.
[0111]
Of the above-mentioned operating conditions (A) and (B), only one of the operating conditions may be adjusted based on the obtained ζ-phase dross amount, or both (A) and (B). The operating conditions may be adjusted. For example, when the amount of ζ-phase dross is excessively small, the bath temperature of the hot-dip galvanizing bath 103 is increased, and the Al of the hot-dip galvanizing bath 103 is increased.The concentration may be lowered. If the ζ-phase dross amount is appropriate, the operating conditions of (A) and (B) may be maintained as they are.
[0112]
A threshold value may be set as an index for determining whether or not the ζ-phase dross amount obtained in the ζ-phase dross amount determination step (S2) is appropriate. In this case, the operating conditions may be adjusted depending on whether or not the obtained ζ-phase dross amount is less than the threshold value. Specifically, the operating conditions may be changed or maintained without being changed depending on whether or not the obtained ζ-phase dross amount is less than the threshold value. For example, if the obtained ζ-phase dross amount is less than the threshold value, it is determined that the ζ-phase dross amount is excessively small, the operating conditions are changed, and the ζ-phase dross amount in the hot-dip galvanizing bath 103 is larger than the current value. Adjust the operating conditions so that the number also increases. Preferably, when the obtained ζ-phase dross amount is less than the threshold value, the operating conditions are changed so that the ζ-phase dross amount is equal to or more than the threshold value. On the other hand, when the obtained ζ-phase dross amount is equal to or greater than the threshold value, it is determined that the ζ-phase dross amount in the hot-dip galvanizing bath 103 is sufficiently large, and the operating conditions are maintained as they are.
[0113]
5. The number of ζ-phase dross per predetermined area, for example, as described above, when the number of ζ-phase dross in the observation field is used as the ζ-phase dross amount, it is converted into the number per unit area (1 cm 2). The threshold value is the number corresponding to 0 pieces / cm 2. For example, when the number of ζ-phase dross in the above-mentioned observation field of view (15 mm × 15 mm = 225 mm 2) is the ζ-phase dross amount, the threshold value is 11.25 (5.0 / cm 2 × 225 mm 2). And. In this case, when the number of ζ-phase dross obtained by the ζ-phase dross amount determination step (S2) is larger than the threshold value (11.25 pieces / 225 mm 2), that is, the unit area (1 cm 2) is converted. When the number is less than 5.0 pieces / cm 2, it is determined that the amount of ζ-phase dross is excessively small, and the operating conditions are adjusted so that the amount of ζ-phase dross in the hot-dip galvanizing bath 103 increases. Preferably, when the ζ-phase dross amount obtained by the ζ-phase dross amount determination step (S2) exceeds the above threshold value (11.25 pieces / 225 mm 2), that is, the obtained ζ-phase dross amount is the unit area. When the number is less than 5.0 pieces / cm 2 when converted by, the number of ζ-phase dross is equal to or greater than the threshold value (11.25 pieces / 225 mm 2) (that is, 5 when converted by unit area). Adjust the operating conditions so that the number is 0 / cm 2 or more). For example, when the ζ-phase dross amount obtained in the ζ-phase dross amount determination step (S2) is less than 5.0 pieces / cm 2 in terms of unit area, the above-mentioned (A) or (B) Implement at least one of the operating conditions to increase the ζ phase dross amount. For example, the bath temperature of the hot-dip galvanizing bath 103 is increased to increase the amount of ζ-phase dross. Further, for example, the Al content of the hot-dip galvanizing bath 103 is reduced to increase the ζ-phase dross amount. The larger the number of ζ-phase dross per predetermined area, the better, and the upper limit is not specified.
[0114]
Preferably, in the operating condition adjusting step (S3), the Fe concentration in the hot-dip galvanizing bath 103 is defined as X (mass%) based on the ζ-phase dross amount obtained in the ζ-phase dross amount determining step (S2). When the Al concentration in the hot-dip galvanizing bath 103 is defined as Y (mass%), the Fe concentration and the Al concentration in the hot-dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (2). ..
0.100 ≤ Y ≤ 0.139 (1)
Y ≤ 0.2945X + 0.1216 (2)
Here, the Al concentration means the Al concentration excluding the Al content contained in the dross among the Al in the hot-dip galvanizing bath 103, and means the so-called Free-Al concentration (mass%). Similarly, the Fe concentration means the Fe concentration in the hot-dip galvanizing bath 103 excluding the Fe content contained in the dross.
[0115]
Equation (1) indicates the range of Al concentration Y (mass%) in the hot-dip galvanizing bath 103. The Al concentration Y in the hot-dip galvanizing bath 103 is related to the amount of top dross, Γ 2-phase dross, and ζ-phase dross produced. When the Al concentration Y is 0.139% or less, the top dross is likely to undergo phase transformation into Γ 2-phase dross and ζ-phase dross. In this case, excessive generation of top dross is suppressed. As a result, it is possible to suppress the formation of surface defects due to the top dross being sandwiched between the sink roll 107 and the steel plate. Therefore, in order to suppress the occurrence of surface defects, the generation of top dross may be suppressed. In order to suppress surface defects, it is sufficient that the Al concentration in the hot-dip galvanizing bath 103 can be maintained at 0.140% or less. However, in the actual operation of the hot-dip galvanizing treatment, there is a possibility that a variation of ± 0.001% may occur at the maximum in the Al concentration control. Therefore, in the formula (1), the upper limit of the Al concentration Y in the hot-dip galvanizing bath 103 is set to 0.139%.
[0116]
From the viewpoint of suppressing the occurrence of surface defects, the lower limit of the Al concentration is not particularly limited. However, it is well known that overalloying can be suppressed in the alloying treatment by increasing the Al concentration in the hot dip galvanizing bath 103 to a certain level or higher. In the formula (1), the lower limit of the Al concentration (the lower limit of the formula (1)) is 0.100%.
[0117]
The lower limit of the Al concentration Y in the hot-dip galvanizing bath 103 may be 0.100%, 0.105%, or 0.110%. Further, the upper limit of the Al concentration Y in the hot-dip galvanizing bath 103 may be 0.139%, 0.135%, 0.130%, or 0. It may be .125%.
[0118]
Equation (2) corresponds to the boundary (phase transformation line) in which the Γ2 phase dross undergoes a phase transformation to the ζ phase dross in the hot dip galvanizing bath 103. If the Al concentration Y in the hot-dip galvanizing bath 103 is higher than the right side of the equation (2), the chemical composition of the hot-dip galvanizing bath 103 is such that the Γ two-phase dross can exist more stably than the ζ-phase dross. It has become. In this case, on the premise that the Al concentration Y in the hot-dip galvanizing bath 103 satisfies the equation (1), the ζ-phase dross is likely to undergo a phase transformation into the Γ2-phase dross. Therefore, in the hot-dip galvanizing bath 103, Γ 2-phase dross is likely to be generated.
[0119]
On the other hand, if the Al concentration Y in the hot-dip galvanizing bath 103 is equal to or less than the right side of the formula (2), that is, if the Al concentration Y and the Fe concentration X satisfy the formula (2), the Al in the hot-dip galvanizing bath 103. On the premise that the concentration Y satisfies the formula (1), the chemical composition of the hot-dip galvanizing bath 103 is in a state where the ζ-phase dross can exist more stably than the Γ2-phase dross. Therefore, the Γ 2-phase dross in the hot-dip galvanizing bath 103 is likely to undergo a phase transformation into a ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the Γ 2-phase dross is likely to decrease.
[0120]
Therefore, in the hot-dip galvanizing treatment described above, if the Al concentration Y and the Fe concentration X in the hot-dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (2), the hot-dip galvanizing bath 103 can be used. , The generation of ζ-phase dross can be promoted, and the amount of Γ2-phase dross having a negative correlation with the amount of ζ-phase dross can be reduced.
[0121]
More preferably, in the operating condition adjusting step (S3), the Fe concentration in the hot-dip galvanizing bath 103 is defined as X (mass%) based on the ζ-phase dross amount obtained in the ζ-phase dross amount determining step (S2). Then, when the Al concentration in the hot-dip galvanizing bath 103 is defined as Y (mass%), the Fe concentration and the Al concentration in the hot-dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (3). do.
0.100 ≤ Y ≤ 0.139 (1)
Y ≤ 0.2945X + 0.1066 (3)
Here, the Al concentration means the Al concentration excluding the Al content contained in the dross among the Al in the hot-dip galvanizing bath 103, and means the so-called Free-Al concentration (mass%). Similarly, the Fe concentration means the Fe concentration in the hot-dip galvanizing bath 103 excluding the Fe content contained in the dross.
[0122]
Equation (3) is an equation that specifies a region having a lower Al concentration than the above-mentioned equation (2). The above equation (2) corresponds to the boundary (phase transformation line) in which the Γ2 phase dross undergoes a phase transformation to the ζ phase dross in the hot dip galvanizing bath 103. Equation (3) is a region in which the ζ-phase dross can exist more stably than the region specified by the equation (2). Therefore, the Γ 2-phase dross in the hot-dip galvanizing bath 103 is more likely to undergo a phase transformation into a ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the Γ 2-phase dross is likely to be further reduced.
[0123]
The Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath can be obtained by the following method. In the hot-dip galvanizing bath 103 of FIG. 2, a sample is taken from within a specific depth range in the depth direction D. More specifically, in the hot-dip galvanizing bath 103 of FIG. 2, a section is formed in a specific depth range in the depth direction D, a specific width range in the width direction W, and a specific length range in the length direction L. A sample is collected from within the specific area (hereinafter referred to as the sampling area). When collecting samples sequentially over time, the sampling positions of the samples shall be the same (within the same sampling area). Cool the collected sample to room temperature. An ICP emission spectrophotometer is used to measure the Fe concentration (% by mass) and the Al concentration (% by mass) in the sample after cooling. The balance other than the Fe concentration and the Al concentration can be regarded as Zn.
[0124]
The Fe concentration obtained by the above-mentioned ICP emission spectrophotometer is a so-called Total-Fe concentration including not only the Fe concentration (Free-Fe concentration) in the hot-dip zinc plating bath but also the Fe concentration in the dross. Similarly, the Al concentration obtained by the above-mentioned ICP emission spectrophotometer is a so-called Total-Al concentration including not only the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath but also the Al concentration in the dross. be. Therefore, the Free-Fe concentration and the Free-Al concentration are calculated using the obtained Total-Fe concentration and Total-Al concentration and a well-known Zn-Fe-Al ternary phase diagram. Specifically, a Zn-Fe-Al ternary phase diagram at the bath temperature at the time of collecting the sample is prepared. 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-Fe concentration and the Total-Al concentration obtained by the ICP emission spectrophotometer are plotted. Then, a tie line (conjugated line) is drawn on the liquid phase line in the Zn-Fe-Al ternary phase diagram from the plotted points. The Fe concentration at the intersection of the liquid phase line and the tie line corresponds to the Free-Fe concentration, and the Al concentration at the intersection of the liquid phase line and the tie line corresponds to the Free-Al concentration. By the above method, the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath can be obtained.
[0125]
[About the more preferable bath temperature of the hot-dip galvanized bath]
The temperature (bath temperature) of the hot-dip galvanizing bath 103 in the above-mentioned hot-dip galvanizing treatment method is preferably 440 to 500 ° C. The dross in the hot-dip galvanizing bath 103 mainly depends on the temperature of the hot-dip galvanizing bath 103 and the Al concentration in the hot-dip galvanizing bath 103, mainly top dross (Fe 2Al 5Zn x), Γ 2-phase dross, and δ 1-phase. And ζ phase transformation to phase dross. Γ Two-phase dross is likely to occur in the region where the bath temperature is low. The ζ phase dross is likely to be generated in the region where the bath temperature is higher than the region where the Γ2 phase dross is generated.
[0126]
Further, if the bath temperature of the hot-dip galvanizing bath 103 is 500 ° C. or lower, it is possible to suppress Zn from evaporating to become a fume. When a fume is generated, the fume adheres to the steel sheet and tends to cause a surface defect (fume defect). Hot-dip galvanizing bath 10 The preferred lower limit of 3 is 460 ° C, more preferably 465 ° C, still more preferably 469 ° C. The preferred upper limit of the hot dip galvanizing bath 103 is 490 ° C, more preferably 480 ° C, still more preferably 475 ° C. It should be noted that the top dross is likely to be generated in the region where the Al concentration is higher than the region where the Γ2 phase dross is generated and the region where the ζ phase dross is generated.
[0127]
As described above, in the hot-dip galvanizing treatment method of the present embodiment, a sample is sampled from the hot-dip galvanizing bath 103 (sample sampling step (S1)), and the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is determined (ζ). Phase dross amount determination step (S2)). Then, the operating conditions of the hot-dip galvanizing treatment are adjusted based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103 (operating condition adjusting step (S3)). By managing the amount of ζ-phase dross, which has a negative correlation with the amount of Γ2-phase dross, the operating conditions can be adjusted so as to suppress the occurrence of dross defects.
[0128]
[Manufacturing method of alloyed hot-dip galvanized steel sheet]
The hot-dip galvanizing treatment method of the present embodiment described above can be applied to a method for manufacturing an alloyed hot-dip galvanized steel sheet (GA).
[0129]
The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present embodiment includes a hot-dip galvanized treatment step and an alloying treatment step. In the hot-dip galvanizing treatment step, the hot-dip galvanizing treatment method described above is carried out on the steel sheet to form a hot-dip galvanizing layer on the surface of the steel sheet. On the other hand, in the alloying treatment step, the alloying treatment is carried out using the alloying furnace 111 shown in FIG. 2 on the steel sheet having the hot-dip galvanized layer formed on the surface by the hot-dip galvanizing treatment step. As the alloying treatment method, it is sufficient to apply a well-known method.
[0130]
The alloyed hot-dip galvanized steel sheet can be manufactured by the above manufacturing process. In the alloyed hot-dip galvanized steel sheet of the present embodiment, the hot-dip galvanized treatment method of the present embodiment described above is adopted. That is, based on the ζ-phase dross amount, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the ζ-phase dross amount. Therefore, the Γ 2-phase dross in the hot-dip galvanized bath 103 is relatively reduced, and as a result, it is possible to suppress the occurrence of dross defects in the manufactured alloyed hot-dip galvanized steel sheet.
[0131]
The method for manufacturing the alloyed hot-dip galvanized steel sheet of the present embodiment may include a hot-dip galvanized treatment step and a manufacturing step other than the alloying treatment step. For example, the method for producing an alloyed hot-dip galvanized steel sheet of the present embodiment may include a tempering rolling step of performing tempering rolling using the tempering rolling mill 30 shown in FIG. 1 after the alloying treatment step. .. In this case, the appearance quality of the surface of the alloyed hot-dip galvanized steel sheet can be further improved. Further, a manufacturing process other than the temper rolling process may be included.
[0132]
[Manufacturing method of hot-dip galvanized steel sheet]
The hot-dip galvanizing treatment method of the present embodiment described above can also be applied to a method for manufacturing a hot-dip galvanized steel sheet (GI).
[0133]
The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes a hot-dip galvanized treatment step. In the hot-dip galvanizing treatment step, the hot-dip galvanizing treatment method described above is carried out on the steel sheet to form a hot-dip galvanizing layer on the surface of the steel sheet. In the method for manufacturing a hot-dip galvanized steel sheet of the present embodiment, the above-mentioned hot-dip galvanized treatment method of the present embodiment is adopted. That is, the operating conditions of the hot-dip galvanizing treatment are adjusted based on the ζ-phase dross amount to increase the ζ-phase dross. Therefore, it is possible to suppress the occurrence of dross defects in the manufactured hot-dip galvanized steel sheet.
[0134]
The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment may include a manufacturing step other than the hot-dip galvanizing treatment step. For example, the method for producing a hot-dip galvanized steel sheet of the present embodiment may include a temper rolling step in which temper rolling is performed using the temper rolling machine 30 shown in FIG. 1 after the hot-dip galvanizing treatment step. In this case, the appearance quality of the surface of the hot-dip galvanized steel sheet can be further improved. Further, a manufacturing process other than the temper rolling process may be included.
Example
[0135]
Hereinafter, the effect of one aspect of the hot-dip galvanizing treatment method of the present embodiment will be described more specifically by way of examples. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. Therefore, the hot-dip galvanizing treatment method of the present embodiment is not limited to this one condition example.
[0136]
In the above-mentioned operating condition adjustment step, the relationship between Fe concentration X and Al concentration Y was investigated.
[0137]
Specifically, a hot-dip galvanizing treatment method was carried out using a hot-dip galvanizing facility having the same configuration as in FIG. Specifically, the Fe concentration X (mass%) and the Al concentration Y (mass%) of the hot-dip galvanizing bath were adjusted as shown in Table 1. As the steel sheet, high-strength steel composed of C: 0.003%, Si: 0.006%, Mn: 0.6%, P: 0.02%, S: 0.01% and the balance: Fe and impurities. Using. This high-strength steel is a so-called difficult-to-alloy material that is relatively difficult to alloy when manufacturing an alloyed hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet was alloyed using an alloying furnace to produce an alloyed hot-dip galvanized steel sheet. The heating temperature in the alloying treatment was constant (510 ° C.) in all the test numbers.
[0138]
In each test number, a sample was taken from the hot-dip galvanizing bath 103 of FIG. 2 in the specific depth range D107 from the upper end to the lower end of the sink roll 107 in the depth direction D. More specifically, in the hot-dip galvanizing bath 103 of FIG. 2, in a specific depth range D107 in the depth direction D, a specific width range in the width direction W, and a specific length range in the length direction L. Samples were collected from within the specific area to be partitioned (hereinafter referred to as the sampling area). In any of the test numbers, about 400 g of a sample was collected from the same sampling area as described above. The collected sample was cooled to room temperature. Using the cooled sample, the chemical composition of the hot-dip galvanized bath of each test number was measured using an ICP emission spectrophotometer. The Fe concentration (mass%) and Al concentration (mass%) obtained by the measurement are Total-Fe concentration (mass%) and Total-Al concentration (mass%). Therefore, using the obtained Total-Fe concentration and Total-Al concentration and a well-known Zn-Fe-Al ternary system state diagram, the Fe concentration (Free-Fe concentration) and melting in the hot-dip galvanizing bath The Al concentration (Free-Al concentration) in the zinc plating bath was calculated. Specifically, a Zn-Fe-Al ternary phase diagram at the bath temperature at the time of collecting the sample was prepared. On a well-known Zn-Fe-Al ternary phase diagram, the points identified from the Total-Fe concentration and the Total-Al concentration obtained by the ICP emission spectrophotometer were plotted. From the plotted points, a tie line (conjugate line) was drawn on the liquid phase line in the Zn-Fe-Al ternary phase diagram to obtain the intersection of the liquid phase line and the tie line. The Fe concentration at the intersection was defined as the Free-Fe concentration (% by mass), and the Al concentration at the intersection was defined as the Free-Al concentration (% by mass). By the above method, the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath were determined. As a result, the Fe concentration in the hot-dip galvanizing bath was in the range of 0.02 to 0.05% by mass in all the test numbers.
[0139]
[table 1]

[0140]
In each test number, the Fe concentration X (mass%) of the hot-dip galvanizing bath is made constant at the value shown in Table 1, and the Al concentration Y (mass%) of the hot-dip galvanizing bath is shown in Table 1. Al was appropriately added over time to adjust the concentration. The transport speed of the steel sheet during the hot-dip galvanizing treatment was constant at all test numbers.
[0141]
Table 1 also describes the values ​​on the right side of equations (2) and (3). Further, it is described whether or not the Fe concentration X (mass%) and the Al concentration Y (mass%) in the hot-dip galvanizing bath satisfy the formulas (1) to (3). For example, when a white circle (○) is described in the column of the formula (2), the Fe concentration X (mass%) and the Al concentration Y (mass%) in the hot-dip galvanizing bath satisfy the formula (2). Is shown. When a cross mark (x) is described in the column of the formula (2), it means that the Fe concentration X (mass%) and the Al concentration Y (mass%) in the hot-dip galvanizing bath do not satisfy the formula (2). show.
[0142]
For each test number, samples were taken from the hot-dip galvanized bath under the operating conditions shown in Table 1. Specifically, about 400 g of a sample was collected from the above-mentioned sampling area. A test piece for ζ-phase dross observation was prepared from the collected sample. The surface to be inspected of the ζ-phase dross observation test piece was 1 cm × 1 cm, and the thickness was 0.5 mm. Using a 100-fold SEM, full-field observation was performed in the field of view (1 cm × 1 cm) of the surface to be inspected, and dross (top dross, bottom dross) was specified based on the contrast. Furthermore, composition analysis using EPMA was performed to classify bottom dross into Γ 2-phase dross, δ 1-phase dross, and ζ-phase dross. Furthermore, the equivalent circle diameter of each of the specified bottom dross (Γ 2-phase dross, δ 1-phase dross, and ζ-phase dross) was determined. Among the above-mentioned ζ-phase dross in a field of view of 1 cm × 1 cm, the number of ζ-phase dross having a circle-equivalent diameter of 10 μm or more was determined. The number of ζ-phase dross having a diameter equivalent to a circle of 10 μm or more (pieces / 1 cm 2) in the observation field was defined as the amount of ζ-phase dross. The amount of ζ-phase dross obtained is shown in Table 1. In this example, no Γ1 phase dross was observed in any of the test numbers.
[0143]
[Dross defect evaluation test]
After performing the hot-dip galvanizing treatment under the operating conditions of each test number, the alloying treatment was carried out under the same conditions for each test number to manufacture an alloyed hot-dip galvanized steel sheet. The surface of the manufactured alloyed hot-dip galvanized steel sheet was visually observed to investigate the presence or absence of dross defects, and the dross defects were evaluated. The criteria for dross defect evaluation are as follows.
A: There were no dross defects (the number of dross defects was 0 / m 2).
B: The number of dross defects is more than 0 and 0.1 / m 2 or less.
C: The number of dross defects is 0.1 / m 2 excess 1 / m 2 or less
[0144]
[Alloying evaluation test for difficult alloying materials]
The chemical composition of the alloyed hot-dip galvanized layer on the surface of the alloyed hot-dip galvanized steel sheet manufactured under the operating conditions of each test number was investigated, and the alloying of the difficult-to-alloy material was evaluated. Specifically, the chemical composition of the alloyed hot-dip galvanized layer was analyzed using an energy dispersive fluorescent X-ray analyzer (EDX-7000) manufactured by Shimadzu Corporation. The Fe content (mass%) contained in the alloyed hot-dip galvanized layer was divided by the Zn content (mass%) contained in the alloyed hot-dip galvanized layer to calculate a numerical value, and the alloying was evaluated. The criteria for alloying evaluation are as follows. When the ratio of Fe content to Zn content was 11% or more, it was judged to be a superalloy.
A: The ratio of Fe content to Zn content is 10% or more and less than 11%.
B: The ratio of Fe content to Zn content is more than 9% and less than 10%.
C: The ratio of Fe content to Zn content is less than 9%
[0145]
[Evaluation results]
With reference to Table 1, in test numbers 1, 2, 5, 6, 8 to 13 in which the ζ-phase dross amount was controlled to 5.0 pieces / cm 2 or more, the dross defect evaluation was A or B, and the dross defect was evaluated. Was able to be suppressed more effectively. In test numbers 1, 2, 5, 6, 8 to 13, the alloying evaluation of the difficult-to-alloy material was further A or B, and even if the hard-to-alloy material was used, the alloying could be promoted more effectively. On the other hand, in the test numbers 3, 4, and 7 in which the ζ-phase dross amount was less than 5.0 pieces / cm 2, the dross defect evaluation and the alloying evaluation of the difficult-to-alloy material were C. Further, referring to test numbers 1 to 13, the larger the amount of ζ-phase dross, the better the dross defect evaluation. That is, the ζ-phase dross amount and the number of dross defects showed a negative correlation.
[0146]
From the above results, it was found that the dross defect can be suppressed by adjusting the operating conditions based on the ζ-phase dross amount.Then, preferably, the threshold value of the ζ-phase dross amount is set to 5.0 pieces / cm 2, and the operating conditions in the hot-dip galvanizing treatment are adjusted so that the ζ-phase dross amount is 5.0 pieces / cm 2 or more. By doing so, it was found that the dross defect can be remarkably suppressed.
[0147]
In the test numbers 1, 2, 5, 6, 8 to 13 satisfying the formulas (1) and (2), the dross defect evaluation was A or B, and the dross defect could be suppressed more effectively. In test numbers 1, 2, 5, 6, 8 to 13, the alloying evaluation of the difficult-to-alloy material was further A or B, and even if the hard-to-alloy material was used, the alloying could be promoted more effectively. Therefore, it was found that adjusting the operating conditions so as to satisfy the formulas (1) and (2) is effective in suppressing dross defects and promoting alloying of difficult-to-alloy materials.
[0148]
In the test numbers 1, 5, 8, 9, 11 and 12 satisfying the formulas (1) and (3), the dross defect evaluation was A, and the dross defect could be suppressed more effectively. In test numbers 1, 5, 8, 9, 11 and 12, the alloying evaluation of the difficult-to-alloy material was further set to A, and even if the hard-to-alloy material was used, alloying could be promoted more effectively. Therefore, it was found that adjusting the operating conditions so as to satisfy the formulas (1) and (3) is more effective in suppressing dross defects and promoting alloying of difficult-to-alloy alloying materials.
[0149]
In test numbers 14 and 16 in which the Al concentration Y in the hot-dip galvanized bath is 0.0990% by mass, the dross defect evaluation is "A", and further, promotion of alloying even for difficult-to-alloy materials. However, overalloying occurred in the production of alloyed hot-dip galvanized steel sheets. Therefore, it was clarified that it is more preferable that the Al concentration Y in the hot-dip galvanizing bath satisfies the formula (1).
[0150]
In test numbers 15 and 17, where the Al concentration Y in the hot-dip galvanizing bath was 0.1410% by mass, the alloying evaluation of the difficult-to-alloy material was "C". Therefore, it was clarified that it is more preferable that the Al concentration Y in the hot-dip galvanizing bath satisfies the formula (1).
[0151]
The embodiment of the present invention has 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-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.
Code description
[0152]
10 Hot-dip galvanizing equipment
101 Hot-dip galvanized pot
103 Hot-dip galvanized bath
107 sink roll
109 gas wiping device
111 alloying furnace
202 Snout
The scope of the claims
[Claim 1]
A hot-dip galvanized treatment method used in the manufacture of hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets.
A sample collection process in which a sample is collected from a hot-dip galvanizing bath containing Al, and
Using the collected sample, the ζ-phase dross amount determination step for determining the ζ-phase dross amount in the hot-dip galvanizing bath, and
It is provided with an operating condition adjustment step for adjusting the operating conditions of the hot-dip galvanizing treatment based on the obtained ζ-phase dross amount.
Hot-dip galvanizing method.
[Claim 2]
The hot-dip galvanizing treatment method according to claim 1.
In the ζ phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area is determined as the amount of ζ-phase dross.
Hot-dip galvanizing method.
[Claim 3]
The hot-dip galvanizing treatment method according to claim 1 or 2.
In the operating condition adjustment process,
Based on the obtained ζ-phase dross amount, at least one of (A) or (B) is carried out to increase the ζ-phase dross amount.
Hot-dip galvanizing method.
(A) Adjust the bath temperature of the hot-dip galvanizing bath.
(B) Adjust the Al concentration of the hot-dip galvanizing bath.
[Claim 4]
The hot-dip galvanizing treatment method according to any one of claims 1 to 3.
In the operating condition adjustment process,
When the obtained ζ-phase dross amount is less than the threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the ζ-phase dross amount.
Hot-dip galvanizing method.
[Claim 5]
The hot-dip galvanizing treatment method according to claim 4.
In the ζ phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area was determined as the amount of ζ-phase dross.
In the operating condition adjustment process,
When the obtained ζ-phase dross amount is less than 5.0 pieces / cm 2 when converted into a unit area (1 cm 2), the operating conditions of the hot-dip galvanizing treatment are adjusted to obtain the ζ-phase dross amount. To increase,
Hot-dip galvanizing method.
[Claim 6]
The hot-dip galvanizing treatment method according to any one of claims 1 to 5.
In the operating condition adjustment process,
When the Fe concentration in the hot-dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass%), the Fe concentration and Al in the hot-dip galvanizing bath are defined. Adjust the concentration to satisfy equations (1) and (2),
Hot-dip galvanizing method.
0.100 ≤ Y ≤ 0.139 (1)
Y ≤ 0.2945X + 0.1216 (2)
[Claim 7]
The hot-dip galvanizing treatment method according to claim 6.
In the operating condition adjustment process,
When the Fe concentration in the hot-dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass%), the Fe concentration and Al in the hot-dip galvanizing bath are defined. Adjust the concentration to satisfy equations (1) and (3),
Hot-dip galvanizing method.
0.100 ≤ Y ≤ 0.139 (1)
Y ≤ 0.2945X + 0.1066 (3)
[Claim 8]
The hot-dip galvanizing treatment method according to any one of claims 1 to 7.
In the hot-dip galvanized pot in which the hot-dip galvanizing bath is stored, a sink roll for contacting the steel sheet immersed in the hot-dip galvanizing bath and changing the traveling direction of the steel sheet up and down is arranged. ,
In the sample collection process,
The sample is collected from the depth range from the upper end to the lower end of the sink roll in the hot dip galvanizing bath in the hot dip galvan pot.
Hot-dip galvanizing method.
[Claim 9]
A hot-dip galvanizing treatment step of forming a hot-dip galvanizing layer on the surface of the steel sheet by carrying out the hot-dip galvanizing treatment method according to any one of claims 1 to 8 on the steel sheet.
The steel sheet having the hot-dip galvanized layer formed on the surface thereof is subjected to an alloying treatment to produce the alloyed hot-dip galvanized steel sheet.
Manufacturing method of alloyed hot-dip galvanized steel sheet.
[Claim 10]
A hot-dip galvanizing treatment step of carrying out the hot-dip galvanizing treatment method according to any one of claims 1 to 8 on a steel sheet to form a hot-dip galvanizing layer on the surface of the steel sheet is provided. A method for manufacturing a hot-dip galvanized steel sheet.