Hot-dip galvanizing process; Production method of hot-dip galvanized steel sheets through alloying using the hot-dip galvanizing process; and the production method of hot-dip galvanized steel sheets using the hot-dip galvanizing process.

Abstract

DEPCT65 What has been provided is a hot-dip galvanizing procedure that can achieve the desired level of durability. The concentrated Al in the hot-dip galvanizing bath provides a stable plating process. This type of hot-dip galvanizing is the hot-dip galvanizing process that is used. For the production of hot-dip galvanized steel sheets or coated steel sheets. Hot-dip galvanizing through alloying is a galvanizing process that includes: The sampling procedure (S1), the dross determination procedure (S2), and the procedure for Adjust the operating conditions (S3) in the sample collection step (S1). Samples were collected from Bath. Hot-dip galvanizing with Al in the determination process results in a dross content (S2). The gamma 2 dross phase and delta 1 dross phase content in the hot-dip galvanizing bath are obtained. Determined by using the examples collected in the operating condition adjustment step (S3). The operating conditions of the hot-dip galvanizing operation were adjusted based on... The specified gamma-2 dross phase and delta-1 dross phase amounts. -----------------------------------------------------------

Description

Hot-dip galvanizing treatment method, method for manufacturing galvannealed hot-dip galvanized steel sheet using the hot-dip galvanizing treatment method, and method for manufacturing galvannealed steel sheet using the hot-dip galvanizing treatment method

[0001] The present invention relates to a hot-dip galvanizing method, a method for producing a galvannealed steel sheet using the hot-dip galvanizing method, and a method for producing a galvannealed steel sheet using the hot-dip galvanizing method.

[0002] Hot-dip galvanized steel sheets (hereinafter also referred to as GI) and galvannealed steel sheets (hereinafter also referred to as GA) are manufactured by the following manufacturing process. First, a steel sheet (base steel sheet) to be subjected to hot-dip galvanizing treatment is prepared. The base steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet. When the base steel sheet is a hot-rolled steel sheet, for example, a pickled hot-rolled steel sheet is prepared. The pickled hot-rolled steel sheet may be subjected to Ni pre-plating treatment as needed to prepare a hot-rolled steel sheet having a Ni layer formed on its surface. A hot-rolled steel sheet subjected to treatments other than those described above may also be prepared. When the base steel sheet is a cold-rolled steel sheet, for example, an annealed cold-rolled steel sheet is prepared. The annealed cold-rolled steel sheet may be subjected to Ni pre-plating treatment as needed to prepare a cold-rolled steel sheet having a Ni layer formed on its surface. A cold-rolled steel sheet subjected to treatments other than those described above may also 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 galvanizing bath to carry out hot-dip galvanizing treatment, thereby producing a hot-dip galvanized steel sheet. When producing a galvannealed hot-dip galvanized steel sheet, the hot-dip galvanized steel sheet is further subjected to a heat treatment in an alloying furnace, thereby producing a galvannealed hot-dip galvanized steel sheet.

[0003] Details of the hot-dip galvanizing treatment in the manufacturing process of hot-dip galvanized steel sheets and galvannealed steel sheets are as follows: The hot-dip galvanizing equipment used for the hot-dip galvanizing treatment includes a molten zinc pot containing a hot-dip galvanizing bath, a sink roll disposed in the hot-dip galvanizing bath, and a gas wiping device.

[0004] In the hot-dip galvanizing process, a steel sheet (base steel sheet) is immersed in a hot-dip galvanizing bath. Then, a sink roll placed in the hot-dip galvanizing bath changes the direction of travel of the steel sheet upward, and the steel sheet is pulled up from the hot-dip galvanizing bath. As the steel sheet is pulled up and moving upward, a wiping gas is blown onto the surface of the steel sheet from a gas wiping device to scrape off excess molten zinc and adjust the coating weight on the steel sheet surface. The hot-dip galvanizing process is carried out in the above manner. Note that, when manufacturing an alloyed hot-dip galvanized steel sheet, the steel sheet with the adjusted coating weight is further charged into an alloying furnace and subjected to alloying treatment.

[0005] Hot-dip galvanizing baths contain a small amount of Al to maintain an appropriate reaction between the steel sheet (base steel sheet) and molten zinc. The Al in the hot-dip galvanizing bath is consumed during the hot-dip galvanizing process. Therefore, an appropriate amount of Al needs to be supplied to the hot-dip galvanizing bath as needed. It is known that the Al concentration in a hot-dip galvanizing bath affects the adhesion of the hot-dip galvanized layer of a hot-dip galvanized steel sheet and the degree of alloying of a galvannealed steel sheet. Therefore, it is preferable to maintain the Al concentration in the hot-dip galvanizing bath constant. In this specification, the term "Al concentration in a hot-dip galvanizing bath" refers to the Al concentration dissolved in the hot-dip galvanizing bath (so-called free-Al concentration). In other words, in this specification, the term "Al concentration in a hot-dip galvanizing bath" refers to the Al concentration dissolved in the hot-dip galvanizing bath (i.e., in the liquid phase), excluding the Al content in the dross (top dross and bottom dross).

[0006] A technology for maintaining a constant Al concentration in a hot-dip galvanizing bath is proposed, for example, in Japanese Patent Laid-Open No. 2018-184630 (Patent Document 1). The Zn—Al alloy supply method described in Patent Document 1 is characterized in that, when a Zn—Al alloy wire is fed into the bath from the bath surface at a predetermined molten zinc bath temperature, the Zn—Al alloy wire is fed at a wire feed rate that allows the Zn—Al alloy wire to completely melt at a depth in the molten zinc bath that allows Al to be uniformly diffused throughout the molten zinc bath. Patent Document 1 states that this enables operation with a more stable Al concentration than when an Al cake is added to adjust the Al concentration.

[0007] JP 2018-184630 A

[0008] Practical Applications of Phase Diagrams in Continuous Galvanizing, Nai-Yong Tang, Journal of Phase Equilibria and Diffusion Vol. 27 No. 5, 2006

[0009] For example, the Al concentration in the hot-dip galvanizing bath can be stabilized by the method described in Patent Document 1. However, it is preferable if the Al concentration in the hot-dip galvanizing bath can be stabilized regardless of the method of supplying Al.

[0010] An object of the present disclosure is to provide a hot-dip galvanizing method capable of stabilizing the Al concentration in a hot-dip galvanizing bath, a method for producing a galvannealed steel sheet using the hot-dip galvanizing method, and a method for producing a galvannealed steel sheet using the hot-dip galvanizing method.

[0011] The hot-dip galvanizing method according to the present disclosure is a hot-dip galvanizing method used in the production of a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet, and includes a sampling step of collecting a sample from a hot-dip galvanizing bath containing Al; and measuring the Γ in the hot-dip galvanizing bath using the collected sample. ２ Phase dross amount and δ １ a dross amount determination step of determining a phase dross amount; ２ Phase dross amount and δ １ and an operating condition adjusting step of adjusting the operating conditions of the hot dip galvanizing process based on the amount of phase dross.

[0012] The method for producing a galvannealed steel sheet according to the present disclosure includes a hot-dip galvanizing treatment step of performing the above-described hot-dip galvanizing treatment method on a steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet, and an alloying treatment step of performing an alloying treatment on the steel sheet with the hot-dip galvanized layer formed on the surface thereof to produce a galvannealed steel sheet.

[0013] The method for producing a hot-dip galvanized steel sheet according to the present disclosure includes a hot-dip galvanizing treatment step of performing the above-described hot-dip galvanizing treatment method on a steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet.

[0014] The hot-dip galvanizing treatment method according to the present disclosure can stabilize the Al concentration in the hot-dip galvanizing bath. Furthermore, the manufacturing method of a galvannealed steel sheet according to the present disclosure can stabilize the Al concentration in the hot-dip galvanizing bath. The manufacturing method of a galvannealed steel sheet according to the present disclosure can stabilize the Al concentration in the hot-dip galvanizing bath.

[0015] FIG. 1 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanizing line used for producing galvannealed steel sheets and hot-dip galvanized steel sheets. FIG. 2 is a side view of the hot-dip galvanizing line in FIG. 1. FIG. 3 is a side view of a hot-dip galvanizing line having a different configuration from that shown in FIG. 2. FIG. 4 is a side view of a hot-dip galvanizing line having a different configuration from that shown in FIG. 2 and FIG. 3. FIG. 5 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanizing line having a different configuration from that shown in FIG. 1. FIG. 6 is a side view of a hot-dip galvanizing line during immersion of an Al ingot. FIG. 7 is a flow chart showing steps of a hot-dip galvanizing treatment method according to this embodiment. FIG. 8 is a diagram showing an example of a photographic image of a part of the observation field of a sample collected in the sample collection step of the hot-dip galvanizing treatment method according to this embodiment.

[0016] As described above, Al in the hot-dip galvanizing bath is consumed by the hot-dip galvanizing treatment. Therefore, Al needs to be supplied to the hot-dip galvanizing bath as needed. Usually, Al is supplied by immersing an Al ingot in the hot-dip galvanizing bath. Al dissolves and diffuses from the Al ingot immersed in the hot-dip galvanizing bath, thereby increasing the Al concentration (free-Al concentration) in the hot-dip galvanizing bath. To reduce the Al concentration in the hot-dip galvanizing bath, the immersion of the Al ingot in the hot-dip galvanizing bath is stopped, and hot-dip galvanizing treatment is performed for a certain period of time.

[0017] For example, when an Al ingot has a rod shape, the Al ingot is gradually immersed in a hot-dip galvanizing solution or an Al pot dissolving Al, with the axial direction of the Al ingot as the up-down direction. The Al pot is connected to a molten zinc pot, and the molten Al is supplied to the hot-dip galvanizing bath. Increasing the immersion speed of the Al ingot increases the amount of Al supplied to the hot-dip galvanizing bath. On the other hand, stopping the immersion of the Al ingot stops the supply of Al to the hot-dip galvanizing bath, and the Al concentration in the hot-dip galvanizing bath gradually decreases. Alternatively, a small Al ingot can be directly introduced into the hot-dip galvanizing bath to supply Al to the hot-dip galvanizing bath. Stopping the introduction of the Al ingot stops the supply of Al to the hot-dip galvanizing bath, and the Al concentration in the hot-dip galvanizing bath gradually decreases.

[0018] Conventionally, the Al concentration in a hot-dip galvanizing bath has been adjusted by adjusting the immersion speed of an Al ingot or the stirring speed of the hot-dip galvanizing bath. In this case, it may be difficult to finely adjust the Al concentration in the hot-dip galvanizing bath. For example, the rate of increase in the Al concentration in the hot-dip galvanizing bath is determined by the rate of dissolution of Al from the Al ingot and the rate of diffusion of Al in the hot-dip galvanizing bath. Furthermore, the rate of decrease in the Al concentration in the hot-dip galvanizing bath is determined by the processing speed of the hot-dip galvanizing process.

[0019] The present inventors considered that the Al concentration in the hot-dip galvanizing bath could be further stabilized by adjusting the Al concentration in the hot-dip galvanizing bath by other methods in addition to adjusting the immersion speed of the Al ingot and the stirring speed of the hot-dip galvanizing bath.

[0020] The inventors have conducted detailed research into Al in hot-dip galvanizing baths and have found the following: In hot-dip galvanizing baths, Al is present in free Al, top-dross, gamma galvanizing baths, and ２ Phase (Γ ２ Phase) Dross and Delta １ phase (δ １It has been found that dross (phase) can exist in four forms: Γ and Γ. As mentioned above, free-Al is Al dissolved in the hot-dip galvanizing bath. More specifically, free-Al refers to the concentration of Al dissolved in the hot-dip galvanizing bath (in the liquid phase), excluding the Al content contained in the dross (top dross and bottom dross). Top dross is an intermetallic compound with a lower specific gravity than the hot-dip galvanizing bath, and is dross that floats to the liquid surface of the hot-dip galvanizing bath. Γ ２ Phase dross and δ １ Phase dross is called bottom dross.

[0021] Gamma ２ The dross phase has a chemical composition of 2% Al, 8% Fe, and 90% Zn, in mass%, and has a face-centered cubic crystal structure. １ The dross phase has a chemical composition consisting of, by mass%, 1% or less of Al, 9% or more of Fe, and 90% or more of Zn, and has a hexagonal crystal structure. ２ Phase dross and δ １ It was found that the phase transformation between the γ phase and the dross phase occurs. ２ The Al content of the phase dross and δ １ The Al content of the dross phase is different from that of the dross phase. ２ Phase dross and δ １ When the phase transformation occurs between the dross and the aluminum, absorption and desorption of aluminum occurs.

[0022] The present inventors have further studied and have found the following: Dross in a hot-dip galvanizing bath is mainly classified into top dross, Γ ２ Phase dross and δ １ The phase transformation occurs in the form of top dross and Γ ２ Phase dross and δ １ The dross phases transform into each other. ２ Phase dross or δ １The tendency of either phase to form dross is affected by the temperature of the hot-dip galvanizing bath and the Al concentration in the hot-dip galvanizing bath (i.e., the free-Al concentration). ２ Phase dross and δ １ There is a region where both Γ and Γ phases exist, depending on the temperature of the hot-dip galvanizing bath and the Al concentration in the hot-dip galvanizing bath. ２ Phase dross is δ １ Phase transformation into dross, δ １ Phase dross is Γ ２ The inventors have found that the Γ phase transforms into dross in the hot dip galvanizing bath. ２ Phase dross and δ １ It was thought that the Al concentration in the hot-dip galvanizing bath could be stabilized by adjusting the operating conditions so that both the Al and dross phases exist in an appropriate content ratio.

[0023] Specifically, when the Al concentration in the hot dip galvanizing bath is high, δ １ Γ, which has a higher Al content than the dross phase ２ The equilibrium shifts in the direction of the formation of dross phase. １ Phase dross is Γ ２ It becomes easier for the phase to transform into dross. １ Phase dross is Γ ２ When the dross phase is transformed into Γ, Al is absorbed from the hot-dip galvanizing bath. As a result, the Al concentration in the hot-dip galvanizing bath decreases. On the other hand, when the Al concentration in the hot-dip galvanizing bath is low, Γ ２ The δ phase has a lower Al content than the dross phase. １ The equilibrium shifts in the direction of the formation of dross phase. ２ Phase dross is δ １ It becomes easier for the phase to transform into dross. ２ Phase dross is δ １ When the alloy transforms into dross, Al is released into the hot-dip galvanizing bath, resulting in an increase in the Al concentration in the hot-dip galvanizing bath.

[0024] That is, the present inventors have discovered a method different from the conventional one, in which the Al concentration (i.e., the free-Al concentration) in the hot-dip galvanizing bath is stabilized by utilizing the absorption and desorption of Al accompanying the phase transformation of the bottom dross. ２ Phase dross amount and δ １ It was thought that the above operation could be carried out by controlling the amount of dross.

[0025] As explained above, the hot-dip galvanizing method of the present embodiment has been completed based on an idea that differs from conventional technical concepts, and specifically, is as follows.

[0026] The hot-dip galvanizing method of [1] is a hot-dip galvanizing method used for producing a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet, and includes a sampling step of taking a sample from a hot-dip galvanizing bath containing Al; and measuring the amount of Γ in the hot-dip galvanizing bath using the taken sample. ２ Phase dross amount and δ １ a dross amount determination step of determining a phase dross amount; ２ The amount of phase dross and the δ １ and an operating condition adjusting step of adjusting the operating conditions of the hot dip galvanizing process based on the amount of phase dross.

[0027] Here, "adjusting the operating conditions of the hot-dip galvanizing treatment" means adjusting the Γ ２ Phase dross amount and δ １ This means adjusting the operating conditions of the hot-dip galvanizing process so that the amount of phase dross can be adjusted. Furthermore, "adjusting the operating conditions of the hot-dip galvanizing process" includes not only the act of changing the operating conditions of the hot-dip galvanizing process, but also the act of maintaining the current operating conditions.

[0028] According to the hot-dip galvanizing treatment method having the above-described configuration, the Γ in the hot-dip galvanizing bath obtained using the sample ２ Phase dross amount and δ １ Based on the amount of phase dross, Γ ２ Phase dross and δ １ If there is a sufficient amount of phase dross and ２ Phase dross and δ１ The operating conditions of the hot-dip galvanizing treatment method are adjusted so that the dross phase exists at an appropriate content ratio. ２ Phase dross amount and δ １ The phases are transformed into each other. ２ Phase dross amount and δ １ The absorption and desorption of Al occurs along with the phase transformation of the dross amount, resulting in a stable Al concentration in the hot-dip galvanizing bath.

[0029] The hot-dip galvanizing method of [2] is the hot-dip galvanizing method according to [1], wherein in the dross amount determining step, Γ per predetermined area is determined using the collected sample. ２ The number of phase dross is calculated by the above Γ ２ The amount of dross is calculated as the amount of δ per specified area. １ The number of phase dross is calculated by the above δ １ The amount of phase dross is calculated.

[0030] Here, the predetermined area is not particularly limited. For example, the predetermined area is determined by using a sample and setting Γ ２ Phase dross and δ １ When observing the phase dross, the area may be the entire area of ​​the observation field, or the area of ​​a unit area (cm ２ ) may also be used.

[0031] The hot-dip galvanizing method according to [3] is the hot-dip galvanizing method according to [1] or [2], wherein in the operation condition adjusting step, the calculated Γ ２ The amount of phase dross and the δ １ The bath temperature of the hot dip galvanizing bath is adjusted based on the amount of dross phase, ２ The amount of phase dross and the δ １ Adjust the amount of dross.

[0032] The bath temperature of the hot dip galvanizing bath is Γ ２ Phase dross to δ １ Phase transformation to dross and δ １ Phase Dross to Γ ２ This is an effective operating condition for switching the phase transformation from sintered steel to dross. ２ Phase dross amount and δ １The bath temperature of the hot dip galvanizing bath is adjusted based on the amount of dross. ２ Phase dross amount and δ １ By adjusting the amount of phase dross, the Al concentration in the hot dip galvanizing bath can be further stabilized.

[0033] The hot-dip galvanizing method according to [4] is the hot-dip galvanizing method according to any one of [1] to [3], wherein in the operation condition adjusting step, the calculated Γ ２ The amount of phase dross and the δ １ Based on the amount of dross, the conveying speed of the steel sheet in the hot-dip galvanizing equipment for carrying out the hot-dip galvanizing treatment is adjusted to adjust the Γ ２ The amount of phase dross and the δ １ Adjust the amount of dross.

[0034] The conveying speed of the steel plate is Γ ２ Phase dross and δ １ This is an effective operating condition for increasing or decreasing the amount of dross, including phase dross. ２ Phase dross amount and δ １ The conveying speed of the steel plate is adjusted based on the amount of dross. ２ Phase dross amount and δ １ By increasing the amount of dross phase, a sufficient amount of Al is absorbed and released, and as a result, the Al concentration in the hot-dip galvanizing bath can be more stabilized.

[0035] The hot-dip galvanizing method according to [5] is the hot-dip galvanizing method according to any one of [1] to [4], wherein in the dross amount determination step, the collected sample is used to determine the amount of dross per unit area (1 cm ２ ) Γ ２ The number of phase dross is calculated by the above Γ ２ Phase dross amount (pieces / cm ２ ) and calculated as the unit area (1 cm ２ ) per δ １ The number of phase dross is calculated by the above δ １ Phase dross amount (pieces / cm ２ ) and in the operation condition adjustment step, ２ The amount of phase dross and the δ １ The amount of phase dross is adjusted to satisfy the formulas (1) and (2).２ Phase dross amount + δ １ Phase dross amount (1) 0.05≦Γ ２ Phase dross amount / δ １ Phase dross amount≦20.00 (2)

[0036] Gamma ２ Phase dross amount and δ １ The total amount of phase dross is 15 pieces / cm ２ If the value is equal to or greater than this, a sufficient amount of Al is more stably absorbed and released to stabilize the Al concentration in the hot-dip galvanizing bath. ２ Phase dross amount and δ １ When the ratio of the phase dross amounts satisfies the formula (2), both an increase and a decrease in the Al concentration in the hot-dip galvanizing bath are more stably suppressed, and therefore, in this case, the Al concentration in the hot-dip galvanizing bath can be more stabilized.

[0037] The hot-dip galvanizing method of [6] is the hot-dip galvanizing method according to any one of [1] to [5], wherein a sink roll is disposed in a molten zinc pot storing the hot-dip galvanizing bath to come into contact with the steel strip immersed in the hot-dip galvanizing bath and change the direction of travel of the steel strip from up to down, and in the sample collection step, the sample is collected from the hot-dip galvanizing bath in the molten zinc pot within a depth range from the upper end to the lower end of the sink roll.

[0038] In this case, the sample is taken from the same depth as the sink roll. Therefore, the adhesion of the galvannealed layer of the galvannealed steel sheet, the alloying degree of the galvannealed layer of the galvannealed steel sheet, and the Γ ２ Phase dross amount and δ １ The correlation with the amount of phase dross can be improved.

[0039] The method for producing a galvannealed steel sheet according to [7] includes a hot-dip galvanizing treatment step of performing the hot-dip galvanizing treatment method according to any one of [1] to [6] on a steel sheet to form a hot-dip galvanized layer on a surface of the steel sheet, and an alloying treatment step of performing an alloying treatment on the steel sheet having the hot-dip galvanized layer formed on the surface thereof to produce the galvannealed steel sheet.

[0040] The method for producing a galvannealed steel sheet of this embodiment employs the above-described hot-dip galvanizing method of this embodiment, which makes it possible to stabilize the Al concentration in the hot-dip galvanizing bath.

[0041] The method for producing a hot-dip galvanized steel sheet according to [8] includes a hot-dip galvanizing treatment step of carrying out the hot-dip galvanizing treatment method according to any one of [1] to [6] on a steel sheet to form a hot-dip galvanized layer on a surface of the steel sheet.

[0042] The method for producing a hot-dip galvanized steel sheet of this embodiment employs the hot-dip galvanizing treatment method of this embodiment described above, which makes it possible to stabilize the Al concentration in the hot-dip galvanizing bath.

[0043] Hereinafter, a hot-dip galvanizing method, a method for manufacturing a galvannealed steel sheet, and a method for manufacturing a galvannealed steel sheet according to the present embodiment will be described with reference to the drawings. In this specification and the drawings, components having substantially the same functions are designated by the same reference numerals, and description thereof will not be repeated.

[0044] [Configuration of Hot-Dip Galvanizing Line] Fig. 1 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanizing line used for producing a galvannealed steel sheet and a hot-dip galvanized steel sheet. Referring to Fig. 1, the hot-dip galvanizing line 1 includes an annealing furnace 20, a hot-dip galvanizing facility 10, and a temper rolling mill (skin-pass mill) 30.

[0045] 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, a steel sheet is supplied to the heating zone of the annealing furnace 20, and annealing is performed on the steel sheet. The annealed steel sheet is cooled in the cooling zone and transported to the hot-dip galvanizing equipment 10. The hot-dip galvanizing equipment 10 is arranged downstream of the annealing furnace 20. In the hot-dip galvanizing equipment 10, a hot-dip galvanizing process is performed on the steel sheet, and a galvannealed steel sheet or a hot-dip galvanized steel sheet is produced. The temper rolling mill 30 is arranged downstream of the hot-dip galvanizing equipment 10. The temper rolling mill 30 lightly reduces the galvannealed steel sheet or the hot-dip galvanized steel sheet produced in the hot-dip galvanizing equipment 10 as necessary to adjust the surface of the galvannealed steel sheet or the hot-dip galvanized steel sheet.

[0046] [Regarding the hot-dip galvanizing equipment 10] Fig. 2 is a side view of the hot-dip galvanizing equipment 10 in Fig. 1. Referring to Fig. 2, the hot-dip galvanizing equipment 10 includes a molten zinc pot 101, a sink roll 107, a support roll 113, a gas wiping device 109, and an alloying furnace 111.

[0047] The interior of the annealing furnace 20, which is disposed upstream of the hot-dip galvanizing equipment 10, is isolated from the air and maintained in a reducing atmosphere. As described above, the annealing furnace 20 heats the continuously transported steel sheet S in the heating zone. This activates the surface of the steel sheet S and adjusts the mechanical properties of the steel sheet S.

[0048] The downstream end of the annealing furnace 20, which corresponds to the outlet side of the annealing furnace 20, has a space in which a turndown roll 201 is disposed. The downstream end of the annealing furnace 20 is connected to the upstream end of a snout 202. The downstream end of the snout 202 is immersed in a hot-dip galvanizing bath 103. The inside of the snout 202 is isolated from the air atmosphere and is maintained in a reducing atmosphere.

[0049] The steel sheet S, whose conveying 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 molten zinc pot 101. A sink roll 107 is arranged inside the molten zinc pot 101. The sink roll 107 has a rotation axis that is parallel to the width direction of the steel sheet S. The width of the sink roll 107 in the axial direction is greater than the width of the steel sheet S. The sink roll 107 comes into contact with the steel sheet S and changes the traveling direction of the steel sheet S to an upward direction in the hot-dip galvanizing equipment 10.

[0050] The support rolls 113 are disposed in the hot-dip galvanizing bath 103 and above the sink roll 107. The support rolls 113 include a pair of rolls. The pair of rolls of the support rolls 113 have rotation axes parallel to the width direction of the steel sheet S. The support rolls 113 sandwich the steel sheet S, the traveling direction of which has been changed upward by the sink roll 107, and support the steel sheet S being transported upward.

[0051] The gas wiping device 109 is disposed 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 facing 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 sheet S. The gas wiping device 109 blows gas onto both surfaces of the steel sheet S pulled out of the hot-dip galvanizing bath 103, thereby scraping off part of the hot-dip galvanizing adhering to both surfaces of the steel sheet S and adjusting the amount of hot-dip galvanizing adhering to the surface of the steel sheet S.

[0052] The alloying furnace 111 is disposed above the gas wiping device 109. The steel sheet S, which has passed through the gas wiping device 109 and been transported upward, passes through the alloying furnace 111, where an alloying treatment is performed on the steel sheet S. The alloying furnace 111 includes, in this order from the inlet side of the steel sheet S to the outlet side, a heating zone, a holding zone, and a cooling zone. The heating zone heats the steel sheet S so that the temperature (sheet temperature) is approximately uniform. The holding zone maintains the sheet temperature of the steel sheet S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to become a galvannealed layer. The cooling zone cools the steel sheet S on which the galvannealed layer has been formed. As described above, the alloying furnace 111 performs the alloying treatment using the heating zone, the holding zone, and the cooling zone. Note that the alloying furnace 111 performs the above-described alloying treatment when producing a galvannealed steel sheet. On the other hand, the alloying furnace 111 does not perform the alloying treatment when producing a galvannealed steel sheet. In this case, the steel sheet S passes through an inactive alloying furnace 111. In this case, "inactive" means, for example, that the alloying furnace 111 is placed online but is turned off (not started). The steel sheet S that has passed through the alloying furnace 111 is transported to the next process by the top rolls 115.

[0053] When producing a hot-dip galvanized steel sheet, the alloying furnace 111 may be moved offline as shown in Fig. 3. In this case, the steel sheet S is transported to the next process by the top roll 115 without passing through the alloying furnace 111.

[0054] In addition, when the hot-dip galvanizing equipment 10 is equipment dedicated to hot-dip galvanized steel sheets, the hot-dip galvanizing equipment 10 does not need to be equipped with the alloying furnace 111 as shown in FIG. 4 .

[0055] [Other Configuration Examples of Hot-Dip Galvanizing Line Equipment] The hot-dip galvanizing line 1 is not limited to the configuration shown in FIG. 1 . 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, as shown in FIG. 5 , a Ni pre-plating facility 40 may be disposed between the annealing furnace 20 and the hot-dip galvanizing facility 10. The Ni pre-plating facility 40 includes a Ni plating cell that stores a Ni plating bath. The Ni plating process is performed by electroplating. Note that the hot-dip galvanizing line 1 shown in FIGS. 1 and 5 includes the annealing furnace 20 and the temper rolling mill 30. However, the hot-dip galvanizing line 1 does not necessarily have to include the annealing furnace 20. Furthermore, the hot-dip galvanizing line 1 does not necessarily have to include the temper rolling mill 30. The hot-dip galvanizing line 1 is required to include at least the hot-dip galvanizing facility 10. The annealing furnace 20 and the temper rolling mill 30 may be disposed as needed. The hot-dip galvanizing line 1 may further include a pickling facility for pickling the steel sheet upstream of the hot-dip galvanizing facility 10, or may further include other facilities in addition to the annealing furnace 20 and the pickling facility. The hot-dip galvanizing line 1 may further include other facilities in addition to the temper rolling mill 30 downstream of the hot-dip galvanizing facility 10.

[0056] [Regarding Adjustment of Al Concentration in Hot-Dip Galvanizing Bath] In the hot-dip galvanizing treatment step in the manufacturing process of a galvannealed steel sheet or a hot-dip galvanized steel sheet using the above-mentioned hot-dip galvanizing line equipment 1, a conventional method for adjusting the Al concentration in the hot-dip galvanizing bath 103 is as follows.

[0057] 6 is a side view of the hot-dip galvanizing equipment 10 while an Al ingot 300 is being immersed. Referring to Fig. 6, the Al ingot 300 is transported onto a molten zinc pot 101 by a well-known transport means. The Al ingot 300 is then lowered by the well-known transport means and immersed in a hot-dip galvanizing bath 103.

[0058] The method for supplying Al into the hot-dip galvanizing bath 103 is not particularly limited. Al may be supplied by an Al ingot 300 as shown in FIG. 6 , or by other methods. For example, Al may be supplied by immersing a wire containing Al in the hot-dip galvanizing bath 103. Alternatively, for example, an Al ingot 300 or the like may be melted in a pot different from the molten zinc pot 101, and then the molten Al may be added to the hot-dip galvanizing bath 103.

[0059] As described above, conventionally, the Al concentration in the hot-dip galvanizing bath 103 has been adjusted by adjusting the immersion speed of the Al ingot 300 in the hot-dip galvanizing bath 103, stirring the hot-dip galvanizing bath 103, or the like. For example, increasing the immersion speed of the Al ingot 300 increases the amount of Al supplied to the hot-dip galvanizing bath 103, thereby increasing the Al concentration in the hot-dip galvanizing bath 103 (i.e., the free-Al concentration). Slowing the immersion speed of the Al ingot 300 reduces the amount of Al supplied to the hot-dip galvanizing bath 103, thereby suppressing an increase in the Al concentration in the hot-dip galvanizing bath 103. Stopping the immersion of the Al ingot 300 stops the supply of Al to the hot-dip galvanizing bath 103, and therefore the Al concentration in the hot-dip galvanizing bath 103 gradually decreases.

[0060] As mentioned above, dross has Γ ２ Phase dross and δ １ It contains phase dross. ２ Phase dross and δ １ The dross phase and the dross phase undergo phase transformation depending on the temperature of the hot-dip galvanizing bath 103 and the Al concentration in the hot-dip galvanizing bath 103. ２ Phase Dross is δ １ The phase transformation into dross occurs, and δ １ Phase Dross is Γ ２ The phase transforms into dross. ２ The Al content of the phase dross and δ １ The Al content of the dross phase is different from that of the Γ ２ Phase dross and δ １ When the phase transformation between the dross and the galvanized dross occurs, absorption and desorption of Al occurs in the dross. ２ Phase dross amount and δ１ The amount of phase dross was calculated and the calculated Γ ２ Phase dross amount and δ １ Adjust the operating conditions based on the phase dross to ２ Phase dross and δ １ If there is a sufficient amount of phase dross and ２ Phase dross and δ １ By adjusting the content ratio of the phase dross to be appropriate, the Al concentration in the hot dip galvanizing bath 103 can be stabilized.

[0061] Therefore, in the hot-dip galvanizing treatment method of the present embodiment, Γ ２ Phase dross amount and δ １ The amount of dross phase is then calculated. ２ Phase dross amount and δ １ The operating conditions of the hot-dip galvanizing treatment are adjusted based on the amount of dross phase. ２ Phase dross amount and δ １ Based on the amount of phase dross, Γ ２ Phase dross amount and δ １ The amount of phase dross is sufficient and Γ ２ Phase dross amount and δ １ The operating conditions of the hot dip galvanizing process are adjusted so that both the amount of dross and the amount of galvanized phase are present. ２ Phase dross and δ １ Due to the phase transformation with the phase dross, Γ ２ Phase dross and δ １ The dross phase absorbs and releases Al in the hot-dip galvanizing bath 103. As a result, the Al concentration in the hot-dip galvanizing bath 103 can be stabilized. ２ Phase dross amount and δ １ Based on the amount of phase dross, Γ ２ Phase dross amount and δ １ The operating conditions of the hot dip galvanizing process are adjusted so that the amount of phase dross has a constant total amount and relative amount.

[0062] The hot-dip galvanizing method of the present embodiment can be applied to a method for producing a galvannealed steel sheet (GA) and a method for producing a galvannealed steel sheet (GI). The hot-dip galvanizing method of the present disclosure will be described in detail below.

[0063] [Regarding the Hot-Dip Galvanizing Treatment Method of the Present Embodiment] [Regarding the Hot-Dip Galvanizing Equipment Used] The hot-dip galvanizing treatment method of the present embodiment uses a hot-dip galvanizing line. The hot-dip galvanizing line has, for example, the configuration shown in Fig. 1 or Fig. 5. However, as described above, the hot-dip galvanizing line used in the hot-dip galvanizing treatment method of the present embodiment may be the equipment shown in Fig. 1 or Fig. 5, or may be the equipment shown in Fig. 1 or Fig. 5 to which other components have been added. Furthermore, a well-known hot-dip galvanizing line having a configuration different from that shown in Fig. 1 or Fig. 5 may also be used.

[0064] [Regarding the steel sheet used in the hot-dip galvanizing treatment] The steel type and size (sheet thickness, sheet width, etc.) of the steel sheet (base steel sheet) used in the hot-dip galvanizing treatment of this embodiment are not particularly limited. The steel sheet may be a known steel sheet applied to a galvannealed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet, depending on the mechanical properties (e.g., tensile strength, formability, etc.) required for the galvannealed steel sheet or hot-dip galvanized steel sheet to be manufactured. A steel sheet used for automobile exterior panels may also be used as the steel sheet (base steel sheet) used in the hot-dip galvanizing treatment.

[0065] The steel sheet (base steel sheet) used in the hot-dip galvanizing treatment of this embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet. For example, the following steel sheets can be used as the base steel sheet: (a) a hot-rolled steel sheet that has been pickled; (b) a hot-rolled steel sheet that has been pickled and then subjected to Ni pre-plating treatment to form a Ni layer on its surface; (c) a cold-rolled steel sheet that has been annealed; and (d) a cold-rolled steel sheet that has been annealed and then subjected to Ni pre-plating treatment to form a Ni layer on its surface. The above (a) to (d) are examples of steel sheets that can be used in the hot-dip galvanizing treatment of this embodiment. The steel sheet used in the hot-dip galvanizing treatment of this embodiment is not limited to the above (a) to (d). A hot-rolled steel sheet or a cold-rolled steel sheet that has been subjected to treatments other than the above (a) to (d) may also be used in the hot-dip galvanizing treatment.

[0066] [Regarding the 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 this embodiment is a plating solution containing a specific concentration of Al, with the remainder consisting of Zn and impurities. If the hot-dip galvanizing bath 103 contains a specific concentration of Al, excessive reaction between Fe and Zn in the bath can be suppressed, and the progress of non-uniform alloying reaction between the steel sheet immersed in the hot-dip galvanizing bath 103 and Zn can be suppressed. Furthermore, the impurity is, for example, Fe, as described below.

[0067] A preferred 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 concentration (by mass) of Al dissolved in the hot-dip galvanizing solution, or the so-called free-Al concentration. When the Al concentration in the hot-dip galvanizing bath 103 is within the range of 0.100 to 0.159% by mass, it is possible to suppress the occurrence of pattern defects other than dross defects, and further to suppress the occurrence of unalloyed parts in the alloying treatment in the manufacturing process of the galvannealed steel sheet.

[0068] As described above, the hot-dip galvanizing bath 103 according to the present disclosure is a coating 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 mass% of Fe eluted from the equipment and steel sheets in the bath. That is, the Fe concentration (mass%) in the hot-dip galvanizing bath 103 is, for example, 0.020 to 0.100 mass%. However, the Fe concentration in the hot-dip galvanizing bath 103 is not limited to the above numerical range. Here, the Fe concentration in the hot-dip galvanizing bath 103 refers to the so-called free-Fe concentration. In other words, in this specification, the "Fe concentration in the hot-dip galvanizing bath" refers to the Fe concentration dissolved in the hot-dip galvanizing bath (i.e., in the liquid phase), excluding the Fe content contained in the dross (top dross and bottom dross).

[0069] The Fe concentration (free Fe concentration) and the Al concentration (free Al concentration) in the hot-dip galvanizing bath can be determined by the following method. A sample is collected from a specific depth range in the depth direction D of the hot-dip galvanizing bath 103 in FIG. 2 . More specifically, in the hot-dip galvanizing bath 103 in FIG. 2 , a sample is collected from a specific region (hereinafter referred to as a sample collection region) defined by a specific depth range in the depth direction D, a specific width range in the width direction W, and a specific length range in the length direction L. When samples are collected sequentially over time, the samples are collected at the same position (within the same sample collection region). The collected samples are cooled to room temperature. The Fe concentration (mass %) and the Al concentration (mass %) in the cooled sample are measured using an ICP atomic emission spectrometer. The remainder other than the Fe concentration and the Al concentration can be considered as Zn.

[0070] The Fe concentration obtained by the above-mentioned ICP optical emission spectrometer is the so-called total Fe concentration, which includes not only the Fe concentration in the hot-dip galvanizing bath (free Fe concentration) but also the Fe concentration in the dross. Similarly, the Al concentration obtained by the above-mentioned ICP optical emission spectrometer is the so-called total Al concentration, which includes not only the Al concentration in the hot-dip galvanizing bath (free Al concentration) but also the Al concentration in the dross. Therefore, the free Fe concentration and 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 is prepared at the bath temperature when the sample was taken. As mentioned above, the Zn-Fe-Al ternary phase diagram is well-known and is disclosed in Figures 2 and 3 of Non-Patent Document 1, etc. Note that Non-Patent Document 1 is a well-known paper among researchers and developers of hot-dip galvanizing baths. Points identified from the total Fe concentration and total Al concentration obtained by an ICP optical emission spectrometer are plotted on a Zn-Fe-Al ternary phase diagram. Then, tie lines (conjugate lines) are drawn from the plotted points to the liquidus in the Zn-Fe-Al ternary phase diagram. The Fe concentration at the intersection of the liquidus and the tie line corresponds to the free Fe concentration, and the Al concentration at the intersection of the liquidus 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 determined.

[0071] [Hot-dip galvanizing treatment method] The hot-dip galvanizing treatment method of this embodiment uses a hot-dip galvanizing bath 103 containing Al. Fig. 7 is a flow diagram showing the steps of the hot-dip galvanizing treatment method of this embodiment. Referring to Fig. 7, the hot-dip galvanizing treatment method of this embodiment includes a sample collection step (S1), a dross amount determination step (S2), and an operating condition adjustment step (S3). Each step will be described in detail below.

[0072] [Sample Collection Step (S1)] In the sample collection step (S1), a portion of the plating solution is collected as a sample from the hot-dip galvanizing bath 103. In the sample collection step (S1), samples are collected over time. "Collecting samples over time" means collecting samples every specific time. The specific time (the period from when a sample is collected until the next sample is collected) may or may not be constant. For example, samples may be collected every hour. Alternatively, the next sample may be collected one hour after the previous sample is collected, and then another sample may be collected 30 minutes later. The specific time is not particularly limited.

[0073] There is no particular limitation on the amount of sample taken from the hot-dip galvanizing bath 103. In the next step, the dross amount determination step (S2), ２ Phase dross amount and δ １ The amount of sample to be collected is not particularly limited as long as it is an amount that allows the amount of phase dross to be determined. For example, the amount of sample to be collected is 100 to 400 g. The collected sample may be brought into contact with a room-temperature metal with high thermal conductivity, and the sample may be rapidly cooled to room temperature and solidified. An example of a room-temperature metal with high thermal conductivity is copper.

[0074] There is no particular limitation on the position where the sample is taken in the hot-dip galvanizing bath 103. For example, referring to Figures 2 to 4, if the hot-dip galvanizing bath 103 is divided into three equal parts D1 to D3 in the depth direction D, the sample may be taken in the uppermost region D1 of the hot-dip galvanizing bath 103, in the middle region D2, or in the lowermost region D3. Γ in the samples taken in each of the regions D1 to D3 ２ Phase dross amount and δ １ However, the amount of dross varies depending on the sampling location. ２ Phase dross amount and δ １It is possible to determine to some extent whether the amount of phase dross is large. Therefore, the location from which the samples are taken is not particularly limited. As shown in Figures 2 to 4, in the hot-dip galvanizing bath 103, the direction parallel to the sheet width direction of the steel sheet S is defined as the width direction W, the depth direction of the hot-dip galvanizing bath 103 is defined as the depth direction D, and the direction perpendicular to the width direction W and the depth direction D is defined as the length direction L. In this case, preferably, samples are taken over time from within a specific region defined by a specific width range in the width direction W, a specific depth range in the depth direction D, and a specific length range in the length direction L. In short, samples are taken over time from the same position (within the specific region) in the hot-dip galvanizing bath 103.

[0075] Preferably, samples are taken from a region as close as possible to the sink roll 107. Specifically, as shown in Figures 2 to 4, samples are taken from within a specific depth range D107 from the top end to the bottom end of the sink roll 107 in the depth direction D of the hot-dip galvanizing bath 103. In other words, the specific depth range is the depth range D107 from the top end to the bottom end of the sink roll 107. The Al concentration (Free-Al concentration) in the vicinity of the steel sheet S is most highly correlated with the effect of the Al concentration on the steel sheet S. Therefore, Γ in the vicinity of the sink roll 107 ２ Phase dross amount and δ １ The amount of dross phase is the most effective index for stabilizing the free-Al concentration. Therefore, it is preferable to take samples from the depth range D107. In this case, Γ is calculated based on the sample taken from the range closest to the surface of the steel sheet S. ２ Phase dross amount and δ １ To determine the amount of dross, Γ ２ Phase dross amount and δ １ This can enhance the correlation between the amount of phase dross and the degree of alloying of the galvannealed layer of the galvannealed steel sheet and the adhesion of the galvannealed layer of the galvannealed steel sheet. It is also preferable to collect samples in the width direction W and the length direction L from regions as close to the sink roll as possible. As described above, samples are collected over time from the same region in the galvannealed bath 103.

[0076] [Dross Amount Determining Step (S2)] In the dross amount determining step (S2), the Γ ２ Phase dross amount and δ １ Calculate the amount of dross phase. ２ Phase dross amount and δ １ The method for determining the amount of phase dross is not particularly limited, and various methods are possible.

[0077] For example, from the sample collected in the sample collection step (S1), Γ ２ Phase dross and δ １ Prepare a test piece for observing the phase dross. ２ Phase dross and δ １ An example of a test piece for observing phase dross is a rectangular parallelepiped (small plate shape) with a surface (test surface) that can secure an observation field of 15 mm x 15 mm and a thickness of 0.5 mm. Using an optical microscope or scanning electron microscope (SEM) with a predetermined magnification, full-field observation is performed in the above observation field (15 mm x 15 mm), and dross within the full field of view is identified. The dross can be identified based on the contrast within the field of view, and the contrast also makes it possible to distinguish between top dross and bottom dross.

[0078] Fig. 8 is an example of a photographic image of a part of the observation field of the sample collected in the sample collection step (S1). Referring to Fig. 8, the photographic image shows the hot-dip galvanized mother phase 200, the top dross 100T, and the bottom dross 100B. 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 their contrast.

[0079] Of the drosses identified within the observation field (15 mm x 15 mm), composition analysis was performed using EPMA on each bottom dross, and Γ ２ Phase dross and δ １ Further, a crystal structure analysis using a TEM is performed on each bottom dross to identify the Γ phase in the above observation field. ２Phase dross and δ １ Alternatively, the type of dross in the field of view (top dross, Γ ２ Phase dross and δ １ Phase dross) may be identified.

[0080] Identified Γ ２ Phase dross and δ １ Based on the phase dross, Γ in the hot dip galvanizing bath 103 ２ Phase dross amount and δ １ The amount of dross in the hot dip galvanizing bath 103 is calculated. ２ Phase dross amount and δ １ The amount of dross can be determined by various indices. For example, Γ per a given area ２ Phase dross and δ １ The number of phase dross is Γ ２ Phase dross amount and δ １ Here, the predetermined area is not particularly limited, and may be, for example, the entire area of ​​the observation field or a unit area (1 cm ２ For example, if the observation field is 15 mm x 15 mm, the observation field (15 mm x 15 mm = 225 mm) may be ２ ) in ２ Number of phase dross (pieces / 225mm ２ ) and δ １ Number of phase dross (pieces / 225mm ２ ) to Γ ２ Phase dross amount and δ １ It may also be the amount of dross per unit area (1 cm ２ ) Γ ２ Number of phase dross (pieces / cm ２ ) and δ １ Number of phase dross (pieces / cm ２ ) to Γ ２ Phase dross amount and δ １ In this case, the amount of Γ in the observation field can be calculated by the following method. ２ Phase dross and δ １ Calculate the number of phase dross. First, identify Γ２ The circle equivalent diameter (μm) and δ of the phase dross １ The circle equivalent diameter (μm) of the phase dross is calculated. ２ Phase dross and δ １ The diameter of a circle when the area of ​​the phase dross is converted into a circle is defined as the equivalent circle diameter (μm). ２ Phase dross and δ １ The circle equivalent diameter (μm) of the dross phase is determined. ２ The number of dross phases and the δ equivalent circle diameter of 10 μm or more １ The number of phase dross is Γ ２ Number of phase dross (pieces / 225mm ２ ) and δ １ Number of phase dross (pieces / 225mm ２ ) is defined as the obtained Γ ２ Number of phase dross (pieces / 225mm ２ ) and δ １ Number of phase dross (pieces / 225mm ２ ) as Γ per unit area ２ Number of phase dross (pieces / cm ２ ) and δ per unit area １ Number of phase dross (pieces / cm ２ ) in this way. ２ ) Γ of 10 μm or more equivalent circle diameter ２ The number of phase dross and the unit area (1 cm ２ ) δ of 10 μm or more of equivalent circle diameter １ The number of phase dross is Γ ２ Phase dross amount and δ １ The observation field is the above area (15 mm × 15 mm = 225 mm ２ ) is not limited to Γ ２ Phase dross and δ １ The upper limit of the circle-equivalent diameter of the phase dross is not particularly limited. ２ Phase dross and δ １ The upper limit of the equivalent circle diameter of the phase dross is, for example, 300 μm.

[0081] In addition, other indicators were used to measure the Γ ２Phase dross amount and δ １ For example, in the above-mentioned observation field, each bottom dross (each Γ ２ Phase dross and each δ １ Then, calculate the area of ​​the bottom dross (phase dross) and the ratio of Γ to the total area of ​​the bottom dross. ２ The ratio of the total area of ​​the phase dross and δ １ The ratio of the total area of ​​the phase dross is defined as Γ ２ Phase dross amount and δ １ The amount of dross may also be used. ２ The ratio of the total area of ​​the phase dross and δ １ The ratio of the total area of ​​the phase dross is defined as Γ ２ Phase dross amount and δ １ The amount of dross in the above-mentioned field of view may also be used. ２ Total area of ​​phase dross (μm ２ ) and δ １ Total area of ​​phase dross (μm ２ ) to Γ ２ Phase dross amount and δ １ Alternatively, the amount of bottom dross (Γ) may be measured by X-ray diffraction on the test surface of the above sample. ２ Phase dross and δ １ The peak intensity of each bottom dross is measured. ２ Phase dross peak intensity and δ １ sum of peak intensities of the phase dross) ２ Ratio of peak intensities of phase dross and δ １ The ratio of the peak intensities of the dross phase is Γ ２ Phase dross amount and δ １ The amount of dross of the Γ phase can also be used. ２ Phase dross and Γ １ Phase dross is not easy to distinguish clearly. However, Γ １ It is believed that almost no dross phase exists in the hot-dip galvanizing bath 103. Therefore, all of the peak intensities obtained at the diffraction angle 2θ=43 to 44° are Γ ２ The peak intensity of the dross phase is regarded as the peak intensity. For example, a Co dry bulb is used as the target for X-ray diffraction measurement. ２Phase dross amount and δ １ The amount of phase dross may also be determined.

[0082] By the above-described method, the Γ in the hot-dip galvanizing bath 103 was measured using the sample collected in the sample collection step (S1). ２ Phase dross amount and δ １ The amount of dross is determined. The dross amount determination step (S2) is preferably performed each time a sample is taken in the sample taking step (S1). Samples are taken over time, and Γ is calculated each time a sample is taken. ２ Phase dross amount and δ １ By determining the amount of dross phase, Γ in the hot dip galvanizing bath 103 ２ Phase dross amount and δ １ It is also possible to grasp the change in the amount of dross over time. Therefore, based on the samples taken over time, it is possible to grasp the change in the amount of dross over time. ２ Phase dross amount and δ １ The amount of phase dross may be determined.

[0083] [Operation Condition Adjustment Step (S3)] In the dross amount determination step (S2), ２ Phase dross amount and δ １ After the amount of dross is determined, the operation condition adjustment step (S3) is carried out.

[0084] In the operation condition adjustment step (S3), Γ in the hot-dip galvanizing bath 103 ２ Phase dross amount and δ １ The operating conditions of the hot dip galvanizing process are adjusted based on the amount of dross. ２ Phase dross amount and δ １ When the amount of dross phase is small, the Γ ２ Phase dross amount and δ １ The operating conditions are adjusted (changed) to increase the amount of dross. ２ Phase dross amount and δ １ If the amount of dross in either phase is excessively large, the operating conditions are adjusted (changed) to reduce the amount of dross in the other phase. ２ Phase dross amount and δ １If the amount of either of the dross phases is excessively small, the operating conditions are adjusted (changed) so as to increase the amount of the smaller dross phase. ２ Phase dross amount and δ １ If the amount of dross phase is appropriate, the current operating conditions may be maintained. ２ Phase dross amount and δ １ There is no particular limitation as long as the amount of dross phase can be adjusted. ２ Phase dross amount and / or δ １ There are no particular limitations on the method for adjusting the operating conditions, as long as the amount of phase dross can be adjusted to increase or decrease.

[0085] Preferably, at least one of the following (A) and (B) is performed as a method for adjusting the operating conditions: (A) Adjusting the bath temperature of the hot-dip galvanizing bath 103; or (B) Adjusting the conveying speed of the steel sheet in the hot-dip galvanizing equipment 10 where the hot-dip galvanizing treatment is performed.

[0086] Regarding (A) above, if the temperature of the hot dip galvanizing bath is increased, Γ ２ Phase Dross is δ １ Therefore, if the temperature of the hot-dip galvanizing bath is increased, the Γ ２ The phase dross is reduced, and instead, δ １ On the other hand, if the temperature of the hot dip galvanizing bath is lowered, the δ １ Phase Dross is Γ ２ Therefore, if the temperature of the hot-dip galvanizing bath is lowered, the δ １ The phase dross is reduced, and instead, Γ ２ The amount of dross increases. ２ Phase dross amount and δ １ Of the phase dross amount, Γ ２ If the amount of dross is excessively large, the temperature of the hot dip galvanizing bath can be increased to reduce the Γ ２ Reduce the amount of phase dross and １ This increases the amount of dross. １ Phase Dross to Γ２ The effect of absorbing Al in the bath (the effect of reducing Al in the bath) due to the phase transformation into dross is maintained. ２ Phase dross amount and δ １ Of the phase dross amount, δ １ If the amount of dross is excessively large, the temperature of the hot dip galvanizing bath can be lowered to reduce δ １ Reduce the amount of phase dross to Γ ２ This increases the amount of dross. ２ Phase dross to δ １ The effect of releasing Al into the bath (the effect of increasing Al in the bath) due to the phase transformation into dross is maintained. ２ If the amount of dross is excessively small, the temperature of the hot dip galvanizing bath is reduced. １ If the amount of dross is excessively small, the temperature of the hot-dip galvanizing bath is increased, thereby stabilizing the Al concentration in the hot-dip galvanizing bath 103.

[0087] Regarding (B) above, if the conveying speed of the steel sheet S in the hot-dip galvanizing equipment 10 is increased, the amount of Fe dissolved from the steel sheet S immersed in the hot-dip galvanizing bath 103 into the hot-dip galvanizing bath 103 increases. More specifically, if the conveying speed of the steel sheet S in the hot-dip galvanizing equipment 10 is increased, the amount of steel sheet S passing through the hot-dip galvanizing bath 103 per unit time increases. As a result, the amount of Fe dissolved from the steel sheet S immersed in the hot-dip galvanizing bath 103 into the hot-dip galvanizing bath 103 increases. At this time, Γ ２ Phase dross and δ １ The amount of dross produced, including the phase dross, increases overall. ２ Phase dross amount and δ １ When the total amount of the phase dross is excessively small, the conveying speed of the steel sheet in the hot-dip galvanizing equipment 10 is increased to reduce the amount of Γ in the hot-dip galvanizing bath 103. ２ Phase dross amount and δ １ The amount of dross can be increased. ２ Phase dross amount and δ １If both of the dross and the dross phases are present in sufficient amounts in the hot-dip galvanizing bath 103, a sufficient amount of Al is absorbed and desorbed more stably to stabilize the Al concentration in the hot-dip galvanizing bath 103. ２ Phase dross amount and δ １ The conveying speed of the steel plate is adjusted based on the amount of dross. ２ Phase dross amount and δ １ By increasing the amount of phase dross, the Al concentration in the hot dip galvanizing bath 103 can be stabilized.

[0088] Among the operating conditions (A) and (B) above, the calculated Γ ２ Phase dross amount and δ １ Based on the amount of phase dross, only one of the operating conditions may be adjusted, or both of the operating conditions (A) and (B) may be adjusted. ２ Phase dross amount and δ １ The total amount of phase dross is excessively small, and δ １ The amount of dross is Γ ２ If the amount is excessively small compared to the amount of dross, the conveying speed of the steel sheet in the hot-dip galvanizing equipment 10 may be increased and the temperature of the hot-dip galvanizing bath may be increased. ２ Phase dross amount and δ １ If the ratio of the amount of sintered ... ２ Phase dross amount and δ １ If the total amount of the phase dross is appropriate, the operating conditions of (B) may be maintained as they are.

[0089] A threshold value may be set as an index for determining whether the dross amount determined in the dross amount determination step (S2) is appropriate. ２ Phase dross amount and δ １ The operating conditions may be adjusted depending on whether the total amount of dross of the phases is less than the threshold value. ２ Phase dross amount and δ １ Depending on whether the total amount of the phase dross is less than the threshold value, the operating conditions may be changed or maintained unchanged. ２ Phase dross amount and δ １ If the total amount of phase dross is less than the threshold, Γ２ Phase dross amount and δ １ The total amount of dross phases was judged to be excessively small, and the operating conditions were changed to reduce the amount of Γ ２ Phase dross amount and δ １ The operating conditions are adjusted so that the amount of dross is increased from the current amount. ２ Phase dross amount and δ １ If the total amount of phase dross is less than the threshold, Γ ２ Phase dross amount and δ １ The operating conditions are changed so that the total amount of dross is equal to or greater than the threshold value. ２ Phase dross amount and δ １ When the total amount of dross phases is equal to or greater than the threshold value, the amount of Γ ２ Phase dross amount and δ １ It is determined that the amount of dross is sufficiently large, and the operating conditions are maintained as they are.

[0090] Γ per given area ２ Number of phase dross and δ １ The number of phase dross, for example, Γ in the observation field, as described above ２ Number of phase dross and δ １ The number of phase dross is Γ ２ Phase dross amount and δ １ When the amount of phase dross is taken as the unit area (1 cm ２ ) when converted to the number of pieces per cm ２ The number of pieces corresponding to Γ ２ Phase dross amount and δ １ In this case, the threshold value of the total amount of dross is set to Γ obtained in the dross amount determination step (S2). ２ Phase dross amount and δ １ The total amount of phase dross is below the threshold value (15 pieces / cm ２ ), then Γ ２ Phase dross amount and δ １ The total amount of dross phases was judged to be excessively small, and the Γ ２ Phase dross amount and δ １ The operating conditions are adjusted so that the total amount of the dross phases increases. ２Phase dross amount and δ １ The total amount of phase dross is set to the threshold value (15 pieces / cm ２ ) when Γ ２ Phase dross amount and δ １ The total amount of phase dross is below the threshold value (15 pieces / cm ２ For example, the operating conditions are adjusted so that the number of pieces is equal to or greater than Γ determined in the dross amount determination step (S2). ２ Phase dross amount and δ １ The total amount of dross phases is 15 pieces / cm when converted into a unit area. ２ When the number is less than Γ, the operating conditions of (B) are carried out. ２ Phase dross amount and δ １ Increase the total amount of phase dross. ２ Number of phase dross and δ １ The greater the number of dross phases, the more the Al concentration in the hot-dip galvanizing bath 103 can be stabilized, so there is no particular upper limit to the number of dross phases. ２ Phase dross amount and δ １ If the total amount of dross of the phases is excessively large, dross defects may occur on the surface of the hot-dip galvanized steel sheet or the galvannealed steel sheet. ２ Number of phase dross and δ １ An upper limit may be set for the number of phase dross. For example, ２ Phase dross amount and δ １ The upper limit of the total amount of phase dross is the unit area (cm ２ ) may be 100 per

[0091] In addition, Γ determined in the dross amount determination step (S2) ２ Phase dross amount and δ １ A threshold value may be set as an index for determining whether the ratio to the amount of phase dross is appropriate. ２ Phase dross amount and δ １ The ratio of the amount of dross to the amount of dross is, for example, δ １ Γ for phase dross amount ２ Ratio of phase dross amount (= Γ ２ Phase dross amount / δ １ In this case, the calculated Γ ２ Phase dross amount and δ１ The operating conditions may be adjusted depending on whether the ratio of the amount of dross to the amount of sintered phase falls within a predetermined range. ２ Phase dross amount and δ １ Depending on whether the ratio of the amount of dross to the amount of sintered phase falls within a predetermined range, the operating conditions may be changed or maintained unchanged. ２ Phase dross amount and δ １ The ratio of the amount of dross to the amount of dross (for example, Γ ２ Phase dross amount / δ １ When the amount of dross) is less than a predetermined lower limit, δ １ The amount of dross is Γ ２ It was judged that the amount of Γ in the hot dip galvanizing bath 103 was excessively large compared to the amount of dross. ２ The amount of dross phase is δ １ The amount of dross in the phase is increased from the present amount, or δ １ The amount of dross is Γ ２ Adjust the operating conditions so that the amount of dross produced is reduced from the current amount. ２ Phase dross amount and δ １ The ratio of the amount of dross to the amount of dross (for example, Γ ２ Phase dross amount / δ １ If the amount of dross is greater than the upper limit, Γ ２ The amount of dross phase is δ １ It was judged that the amount of Γ in the hot dip galvanizing bath 103 was excessively large compared to the amount of dross. ２ The amount of dross phase is δ １ The amount of dross is reduced from the present amount, or δ １ The amount of dross is Γ ２ Adjust the operating conditions so that the amount of dross increases from the current amount. ２ Phase dross amount and δ １ The ratio of the amount of dross to the amount of dross (for example, Γ ２ Phase dross amount / δ １ When the amount of dross (phase dross amount) is within a predetermined range, Γ ２ Phase dross amount and δ １ The ratio of the slag to the amount of dross is judged to be appropriate, and the operating conditions will be maintained as they are.

[0092] Γ per given area２ Number of phase dross and δ １ The number of phase dross, for example, Γ in the observation field, as described above ２ Number of phase dross and δ １ The number of phase dross is Γ ２ Phase dross amount and δ １ When the amount of phase dross is taken as the unit area (1 cm ２ ) when converted to the number of pieces per １ Γ for the number of phase dross ２ The ratio of the number of phase dross (=Γ ２ Number of Phase Dross / δ １ The range in which the number of phase dross) is 0.05 to 20.00 is defined as Γ ２ Phase dross amount and δ １ In this case, the ratio of the amount of dross to the amount of dross determined in the dross amount determination step (S2) is set to an appropriate range. ２ Phase dross amount and δ １ Ratio to the amount of phase dross (Γ ２ Phase dross amount / δ １ When the amount of dross of the sintered phase is less than the lower limit (0.05), δ １ The amount of dross is Γ ２ It was judged that the amount of δ in the hot dip galvanizing bath 103 was excessively large compared to the amount of dross. １ The amount of dross is Γ ２ To reduce the amount of dross, or ２ The amount of dross phase is δ １ The operating conditions are adjusted so that the dross amount increases. ２ Phase dross amount and δ １ Ratio to the amount of phase dross (Γ ２ Phase dross amount / δ １ When the amount of dross is greater than the upper limit (20.00), Γ ２ The amount of dross phase is δ １ It was judged that the amount of Γ in the hot dip galvanizing bath 103 was excessively large compared to the amount of dross. ２ The amount of dross phase is δ １ To reduce the amount of dross, or δ １ The amount of dross is Γ ２ The operating conditions are adjusted so that the amount of dross increases.

[0093] Preferably, δ determined in the dross amount determination step (S2) １ Γ for the number of phase dross ２ The ratio of the number of phase dross (Γ ２ Phase dross amount / δ １ When the amount of dross) is less than the lower limit (0.05), δ １ Γ for the number of phase dross ２ The operating conditions are adjusted so that the ratio of the number of dross phases is equal to or greater than the lower limit (0.05). １ Γ for the number of phase dross ２ When the ratio of the number of dross phases is less than the lower limit (0.05), the operating conditions of (A) are adjusted to δ １ Γ for the number of phase dross ２ The ratio of the number of dross phases is increased. １ Γ for the number of phase dross ２ When the ratio of the number of phase dross is greater than the upper limit (20.00), δ １ Γ for the number of phase dross ２ The operating conditions are adjusted so that the ratio of the number of dross phases is equal to or less than the upper limit (20.00). １ Γ for the number of phase dross ２ When the ratio of the number of dross phases is more than the upper limit (20.00), the operating conditions of (A) are adjusted to obtain Γ ２ δ for the number of phase dross １ Reduces the ratio of Phase Dross counts.

[0094] [More Preferred Bath Temperature of Hot-Dip Galvanizing Bath] The temperature of the hot-dip galvanizing bath (bath temperature) in the above-described hot-dip galvanizing treatment method is preferably 440 to 500° C. The dross in the hot-dip galvanizing bath 103 is mainly divided into top dross, Γ ２ Phase dross and δ １ The phase transforms into dross. ２ Dross phase is more likely to form in the low bath temperature region. １ Dross phase is more likely to form in regions with high bath temperatures.２ Phase dross and δ １ If the temperature is adjusted to a range in which both the Al phase and the Al phase dross are stably produced, the effect of stabilizing the Al concentration in the hot dip galvanizing bath is enhanced.

[0095] Furthermore, if the bath temperature of the hot-dip galvanizing bath is 500°C or less, evaporation of Zn and generation of fumes can be suppressed. When fumes are generated, the fumes tend to adhere to the steel sheet and cause surface defects (fume defects). The preferred lower limit of the bath temperature of the hot-dip galvanizing bath is 460°C, more preferably 465°C, and even more preferably 469°C. The preferred upper limit of the bath temperature of the hot-dip galvanizing bath is 490°C, more preferably 480°C, and even more preferably 475°C. The top-dross is Γ ２ Phase dross formation region and δ １ It is more likely to form in regions with higher Al concentrations than in regions where phase dross forms.

[0096] As described above, in the hot-dip galvanizing treatment method of the present embodiment, a sample is taken from the hot-dip galvanizing bath (sampling step (S1)), and Γ in the hot-dip galvanizing bath 103 is measured. ２ Phase dross amount and δ １ The amount of dross in the hot dip galvanizing bath 103 is then determined (dross amount determination step (S2)). ２ Phase dross amount and δ １ Based on the amount of phase dross, the operating conditions of the hot dip galvanizing treatment are adjusted (operating condition adjusting step (S3)). ２ Phase dross amount and δ １ By controlling the total amount and ratio of the phase dross amounts, the Al concentration in the hot dip galvanizing bath can be stabilized.

[0097] [Method for Manufacturing Galvannealed Steel Sheet] The hot-dip galvanizing method of the present embodiment described above is applicable to a method for manufacturing a galvannealed steel sheet (GA).

[0098] The method for producing a galvannealed steel sheet according to this embodiment includes a hot-dip galvanizing treatment step and an alloying treatment step. In the hot-dip galvanizing treatment step, the steel sheet is subjected to the above-described hot-dip galvanizing treatment method to form a hot-dip galvanized layer on the surface of the steel sheet. Meanwhile, in the alloying treatment step, the steel sheet on whose surface the hot-dip galvanized layer has been formed by the hot-dip galvanizing treatment step is subjected to alloying treatment using an alloying furnace 111 shown in Fig. 2. Any known method may be used as the alloying treatment method.

[0099] The galvannealed steel sheet can be manufactured by the above-described manufacturing process. The galvannealed steel sheet of this embodiment employs the above-described hot-dip galvanizing method of this embodiment. ２ Phase dross amount and δ １ Based on the amount of dross, the operating conditions of the hot dip galvanizing process are adjusted to obtain Γ ２ Phase dross amount and δ １ The total amount and ratio of the amounts of dross of the phases are adjusted. Therefore, the Al concentration in the hot-dip galvanizing bath 103 is stabilized. As a result, the degree of alloying of the galvannealed layer of the manufactured galvannealed steel sheet is stabilized. If the degree of alloying of the galvannealed layer is stabilized, the appearance of the galvannealed layer becomes more beautiful.

[0100] The method for producing a galvannealed steel sheet according to this embodiment may include other production steps in addition to the hot-dip galvanizing treatment step and the alloying treatment step. For example, the method for producing a galvannealed steel sheet according to this embodiment may include a temper rolling step in which temper rolling is performed using the temper rolling mill 30 shown in FIG. 1 after the alloying treatment step. In this case, the appearance quality of the surface of the galvannealed steel sheet can be further improved. Furthermore, the method may include other production steps in addition to the temper rolling step.

[0101] [Method for manufacturing 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 hot-dip galvanized steel sheet (GI).

[0102] The method for manufacturing a hot-dip galvanized steel sheet according to this embodiment includes a hot-dip galvanizing treatment step. In the hot-dip galvanizing treatment step, the above-described hot-dip galvanizing treatment method is applied to the steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet. The method for manufacturing a hot-dip galvanized steel sheet according to this embodiment employs the above-described hot-dip galvanizing treatment method according to this embodiment. ２ Phase dross amount and δ １ Based on the amount of dross, the operating conditions of the hot dip galvanizing process are adjusted to obtain Γ ２ Phase dross amount and δ １ The total amount and ratio of the amounts of dross of the phases are adjusted. Therefore, the Al concentration in the hot-dip galvanizing bath 103 is stabilized. As a result, the adhesion of the produced hot-dip galvanized layer is stabilized. Stable adhesion of the hot-dip galvanized layer improves the workability of the hot-dip galvanized steel sheet.

[0103] The method for producing a hot-dip galvanized steel sheet according to this embodiment may include other production steps in addition to the hot-dip galvanizing treatment step. For example, the method for producing a hot-dip galvanized steel sheet according to this embodiment may include a temper rolling step in which temper rolling is performed using the temper rolling mill 30 shown in FIG. 1 after the hot-dip galvanizing treatment step. In this case, the appearance quality of the surface of the hot-dip galvanized steel sheet can be further improved. Furthermore, the method may include other production steps in addition to the temper rolling step.

[0104] The effects of one aspect of the hot-dip galvanizing method of this embodiment will be described in more detail below using examples. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of this embodiment. Therefore, the hot-dip galvanizing method of this embodiment is not limited to this one example of conditions.

[0105] In the above-mentioned operation condition adjustment process, Γ ２ Phase dross amount and δ １ The relationship between the amount of phase dross and the Al concentration in the hot dip galvanizing bath was investigated.

[0106] Specifically, a hot-dip galvanizing treatment method was carried out using a hot-dip galvanizing facility having the same configuration as that shown in FIG. ２ Phase dross amount and δ １The hot-dip galvanizing treatment was carried out under conditions with different amounts of dross phase, and the Al concentration in the hot-dip galvanizing bath was investigated. ２ Phase dross amount (pieces / cm ２ ) and δ １ Phase dross amount (pieces / cm ２ ) are shown in Table 1. The steel sheet used was a steel sheet for automobile exterior panels (cold-rolled steel sheet). The bath temperature of the hot-dip galvanizing bath was Γ ２ Phase dross amount (pieces / cm ２ ) and δ １ Phase dross amount (pieces / cm ２ The temperature was adjusted appropriately within the range of 440 to 500°C so that the Γ ２ Phase dross amount (pieces / cm ２ ) and δ １ Phase dross amount (pieces / cm ２ In each test number, the bath temperature of the hot dip galvanizing bath and the conveying speed of the steel sheet were constant.

[0107] For each test number, a sample was collected from a specific depth range D107 in the depth direction D of the hot-dip galvanizing bath 103 shown in FIG. 2 , from the top to the bottom of the sink roll 107. More specifically, in the hot-dip galvanizing bath 103 shown in FIG. 2 , a sample was collected from a specific region (hereinafter referred to as the sample collection region) defined by 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. For each test number, approximately 400 g of sample was collected from the same sample collection region. The collected samples were cooled to room temperature. Using the cooled samples, the chemical composition of the hot-dip galvanizing bath for each test number was measured using an ICP atomic emission spectrometer. The Fe concentration (mass%) and Al concentration (mass%) obtained by the measurement are the total Fe concentration (mass%) and the total Al concentration (mass%). Therefore, the Fe concentration (free-Fe concentration) in the hot-dip galvanizing bath was 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 was prepared at the bath temperature when the sample was taken. Points identified from the total-Fe concentration and total-Al concentration obtained by an ICP optical emission spectrometer were plotted on the well-known Zn-Fe-Al ternary phase diagram. A tie line (conjugate line) was drawn from the plotted points to the liquidus in the Zn-Fe-Al ternary phase diagram, and the intersection between the liquidus and the tie line was determined. The Fe concentration at the intersection was defined as the free-Fe concentration (mass%). Using the above method, the Fe concentration (free-Fe concentration) in the hot-dip galvanizing bath was determined. As a result, the Fe concentration in the hot dip galvanizing bath was within the range of 0.020 to 0.050 mass % for all test numbers.

[0108]

[0109] For each test number, a sample was taken from the hot dip galvanizing bath under the operating conditions shown in Table 1. Specifically, a sample weighing about 400 g was taken from the above-mentioned sample taking area. ２ Phase dross and δ １ Test pieces for observing the phase dross were prepared. ２Phase dross and δ １ The test surface of the test piece for phase dross observation was 15 mm × 15 mm and 0.5 mm thick. A 100x SEM was used to perform a full-field observation of the test surface (15 mm × 15 mm), and the dross (top dross, bottom dross) was identified based on the contrast. Furthermore, a composition analysis was performed using an EPMA to identify the bottom dross as Γ ２ Phase dross and δ １ The phase dross was further classified. ２ Phase dross and δ １ The circle equivalent diameter of the phase dross was determined. ２ Phase dross and δ １ Among the phase dross, Γ with a circle equivalent diameter of 10 μm or more ２ Number of phase dross and δ １ The number of dross phases was counted. ２ Number of phase dross (pieces / 225mm ２ ) as Γ per unit area ２ Number of phase dross (pieces / cm ２ ) and Γ ２ The amount of dross was determined as the amount of δ phase. １ Number of phase dross (pieces / 225mm ２ ) per unit area δ １ Number of phase dross (pieces / cm ２ ) and δ １ The results are shown in Table 1.

[0110] [Test to Evaluate Inhibition of Increase in Al Concentration in the Bath] The Al concentration in the hot-dip galvanizing bath was measured during the hot-dip galvanizing treatment under the operating conditions of each test number. The Al concentration in the hot-dip galvanizing bath was determined by the following method. The chemical composition of the hot-dip galvanizing bath was measured using an ICP optical emission spectrometer. The Fe concentration (mass%) and Al concentration (mass%) obtained by the measurement were the total Fe concentration (mass%) and total Al concentration (mass%). Therefore, the Al concentration (free Al concentration) in the hot-dip galvanizing bath was 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 was prepared at the bath temperature when the sample was taken. Points identified from the total Fe concentration and total Al concentration obtained by the ICP optical emission spectrometer were plotted on the well-known Zn-Fe-Al ternary phase diagram. From the plotted points, tie lines (conjugate lines) were drawn to the liquidus line in the Zn-Fe-Al ternary phase diagram, and the intersection point between the liquidus line and the tie line was determined. The Al concentration at the intersection point was defined as the free-Al concentration (mass%). The Al concentration (free-Al concentration) in the hot-dip galvanizing bath was determined by the above method.

[0111] During the hot-dip galvanizing treatment under the operating conditions of each test number, an Al ingot was immersed at a constant speed. The increase in Al concentration in the hot-dip galvanizing bath per unit time, calculated from the total amount of Al immersed, was determined and defined as the Al increment (ingot) (Al concentration (mass%) / unit time). The Al concentration in the hot-dip galvanizing bath during the hot-dip galvanizing treatment under the operating conditions of each test number was measured over time using the method described above, and the increase in Al concentration per unit time was determined and defined as the Al increment (bath) (Al concentration (mass%) / unit time). The ratio of the Al increment (bath) to the Al increment (ingot) was calculated to evaluate the suppression of the increase in the Al concentration in the hot-dip galvanizing bath. The evaluation criteria were as follows. The results are shown in Table 1. A: The ratio of the Al increment (bath) to the Al increment (ingot) was 30% or less. B: The ratio of the Al increment (bath) to the Al increment (ingot) was more than 30%.

[0112] [Test to Evaluate Suppression of Reduction in Al Concentration in Bath] The Al concentration in the hot-dip galvanizing bath was measured during the hot-dip galvanizing treatment under the operating conditions of each test number. The Al concentration in the hot-dip galvanizing bath was measured using ICP (inductively coupled plasma atomic emission spectroscopy). During the hot-dip galvanizing treatment under the operating conditions of each test number, the immersion of the Al ingot was stopped for a certain period of time before the hot-dip galvanizing treatment. The Al concentration in the hot-dip galvanized layer formed on the resulting hot-dip galvanized steel sheet was measured. The decrease in the Al concentration in the hot-dip galvanized bath per unit time was calculated from the Al concentration in the hot-dip galvanized layer and the conveying speed of the steel sheet, and this was defined as the Al decrease (the amount of Al carried out from the steel sheet) (Al concentration (mass%) / unit time). Here, the amount of Al carried out from the steel sheet corresponds to the amount of Al that was reduced from the hot-dip galvanizing bath in the form of being included in the hot-dip galvanized layer as the hot-dip galvanizing treatment progressed. The Al concentration in the hot-dip galvanizing bath was measured continuously during the hot-dip galvanizing treatment under the operating conditions for each test number, and the amount of decrease in Al concentration per unit time was determined, which was defined as the Al decrease (in the bath) (Al concentration (mass%) / unit time). The ratio of the Al decrease (in the bath) to the Al decrease (amount of Al carried out from the steel sheet) was calculated, and the suppression of decrease in the Al concentration in the hot-dip galvanizing bath was evaluated. The evaluation criteria were as follows. The results are shown in Table 1. A: The ratio of the Al decrease (in the bath) to the Al decrease (amount of Al carried out from the steel sheet) is 30% or less. B: The ratio of the Al decrease (in the bath) to the Al decrease (amount of Al carried out from the steel sheet) is more than 30%.

[0113] In Table 1, in the column for Al concentration stabilization, test numbers that received an A rating in the above-mentioned test for evaluating inhibition of increase in Al concentration in the bath and also received an A rating in the above-mentioned test for evaluating inhibition of decrease in Al concentration in the bath are marked with an A. Test numbers that received a B rating in the above-mentioned test for evaluating inhibition of increase in Al concentration in the bath and / or the above-mentioned test for evaluating inhibition of decrease in Al concentration in the bath are marked with a B.

[0114] [Evaluation Results] Referring to Table 1, in test numbers 5 to 7 and 9 to 11, Γ ２ Phase dross amount and δ １ The total amount of phase dross is 15 pieces / cm ２ is equal to or greater than δ １ Γ for phase dross amount２ The ratio of the amount of dross of the phases was controlled to 0.05 to 20.00. Therefore, in Test Nos. 5 to 7 and 9 to 11, both the increase in the Al concentration in the hot-dip galvanizing bath and the decrease in the Al concentration in the hot-dip galvanizing bath were suppressed, and the Al concentration in the hot-dip galvanizing bath was stabilized.

[0115] From the above results, Γ ２ Phase dross amount and δ １ It has been found that the Al concentration in the hot-dip galvanizing bath can be stabilized by adjusting the operating conditions based on the amount of dross phase. ２ Phase dross amount and δ １ The threshold for the total amount of phase dross is 15 pieces / cm ２ and δ １ Γ for phase dross amount ２ The ratio of the phase dross amount is set to a range of 0.05 to 20.00 as an appropriate content ratio, and Γ ２ Phase dross amount and δ １ The total amount of phase dross is 15 pieces / cm ２ is equal to or greater than δ １ Γ for phase dross amount ２ It was found that the Al concentration in the hot-dip galvanizing bath can be stabilized by adjusting the operating conditions in the hot-dip galvanizing process so that the ratio of the phase dross amount falls within the range of 0.05 to 20.00.

[0116] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

[0117] 10 Hot-dip galvanizing equipment 101 Molten zinc pot 103 Hot-dip galvanizing bath 107 Sink roll 109 Gas wiping device 111 Alloying furnace 202 Snout

Claims

DEPCT651. A hot-dip galvanizing procedure to be used for the production of hot-dip galvanized steel sheets or hot-dip galvanized steel sheets through alloy formation, consisting of a sample collection procedure from the hot-dip galvanizing bath in which Al is determined; a procedure to determine the dross content, gamma phase 2 dross content, and delta phase 1 dross content in the hot-dip galvanizing bath using the collected samples; and a procedure to adjust the operating conditions of the hot-dip galvanizing procedure based on the determined gamma phase 2 dross content and delta phase 1 dross content.

2. A hot-dip galvanizing procedure according to claim 1 in which, in the dross content determination procedure using the collected samples, the number of gamma phase 2 dross particles per predetermined area is determined as the gamma phase 2 dross content and the number of delta phase 1 dross particles per predetermined area is determined as the delta phase 1 dross content.3.

4. Any one of the hot-dip galvanizing operations under Relief 1 to 3 in which, during the conditioning step, the gamma phase 2 and delta phase 1 dross-phase quantities are adjusted by adjusting the conveying speed of the hot-dip galvanizing bath based on specified gamma phase 2 and delta phase 1 dross-phase quantities. 5.In any of the hot-dip zinc plating operations under claims 1 to 4, where in the step of determining the dross quantity by using collected samples, the number of gamma phase 2 dross particles per unit area (1 sq. cm.) is determined as the gamma phase 2 dross quantity (sq. cm.), and the number of delta phase 1 dross particles per unit area (1 sq. cm.) is determined as the delta phase 1 dross quantity ( / sq. cm.), and in the step of adjusting the operating conditions, the dross quantity of gamma phase 2 and delta phase 1 are true to the formulas (1) and (2)15 less than or equal to gamma phase 2 dross quantity + delta phase 1 dross quantity(1)0.05 less than or equal to gamma phase 2 dross quantity / delta phase 1 dross quantity less than or equal to 20.00(2)6.In any of the hot-dip galvanizing operations under claims 1 to 5, whereby a submersible roller in contact with a steel sheet submerged in the hot-dip galvanizing bath and altering the forward movement of the steel sheet to an up or downward direction is placed in the molten zinc bath containing the hot-dip galvanizing bath, and during the collection phase, samples are collected from the depth extending from the top end to the bottom end of the submersible roller of the hot-dip galvanizing bath in the molten zinc bath.7.The manufacturing method of hot-dip galvanized steel sheets through alloyation involves the hot-dip galvanizing operation of any one of the hot-dip galvanizing operations under claims 1 to 6 on the steel sheet to form a hot-dip galvanizing layer on the surface of the steel sheet, and the alloyation operation of the alloyation operation on the surface of the steel sheet on which the hot-dip galvanizing layer has been formed, in order to produce hot-dip galvanized steel sheets through alloyation.