METHOD FOR THE PRODUCTION OF GALVANIZED STEEL SHEET, AND CONTINUOUS HOT-DIP GALVANIZING APPARATUS

Abstract

A method is provided for the production of an annealed galvanized steel sheet, by which high coating adhesion and a favorable coating appearance can be obtained when a hot-dip galvanized coating is applied to a steel sheet having a Si content of 0.2% by mass or more, and the dew point of the homogenization zone atmosphere can be rapidly changed so as to suppress the occurrence of oxidized metal adhesion defects when a hot-dip galvanized coating is subsequently applied to a steel sheet having a Si content of less than 0.2% by mass. When the steel sheet passing through the homogenization zone is of a type of steel containing 0.2% by mass or more of Si, both dry gas and humidified gas are supplied to the homogenization zone, wherein the humidified gas is supplied only from the humidified gas supply orifice located in a second part of the homogenization zone among a plurality of humidified gas supply orifices, wherein the second part of the homogenization zone is determined by considering a sheet pass speed V and a target temperature T on the outlet side of the homogenization zone.

Description

METHOD FOR THE PRODUCTION OF GALVANIZED STEEL SHEET, AND CONTINUOUS HOT-DIP GALVANIZING APPARATUS FIELD OF INVENTION This disclosure relates to a continuous hot-dip galvanizing apparatus comprising an annealing furnace in which a heating zone, a homogenizing zone and a cooling zone are arranged in the specified order, a hot-dip galvanizing apparatus located downstream of the cooling zone, and an alloying line located downstream of the hot-dip galvanizing apparatus, and a method for producing an annealed galvanized steel sheet using the apparatus. BACKGROUND OF THE INVENTION In recent years, the demand for high tensile strength steel sheets, which contribute to weight reduction in structures and similar applications, has been increasing in the automotive, household appliance, and construction materials sectors. It is known that a high tensile strength steel material can produce a steel sheet with good hole expansion capacity by containing silicon (Si), and a steel sheet with good ductility where retained austenite (y) forms readily by containing silicon (Si) or aluminum (Al), for example. However, when producing a galvano-annealed steel sheet using, as a base material, a high-tensile-strength steel sheet containing a large amount of Si (specifically, 0.2% by mass or more), the following problem arises. The galvano-annealed steel sheet is produced by heat-annealing a base material steel sheet at a temperature of approximately 600 °C to 900 °C in a reducing or non-oxidizing atmosphere, then subjecting the steel sheet to hot-dip galvanizing treatment, and further heat-alloying the galvanizing. Here, silicon (Si) in steel is an oxidizable element, selectively oxidizing even in the typically used reducing or non-oxidizing atmospheres. This oxidation concentrates on the surface of the steel sheet to form oxides. These oxides decrease wettability with molten zinc during the galvanizing process, leading to undercoating. Therefore, as the concentration of Si in the steel increases, wettability rapidly decreases, and undercoating frequently occurs. Even when undercoating is avoided, poor coating adhesion remains a problem. Furthermore, if the Si in the steel selectively oxidizes and concentrates on the surface of the steel sheet, it would significantly delay alloying in the process after hot-dip galvanizing, severely impacting productivity. In view of these problems, JP 2016-017192 A (PTL 1) describes a method for producing a galvano-annealed steel sheet, which includes a step of transferring a steel sheet to a heating zone having a direct flame furnace (DFF), a homogenization zone, and a cooling zone in the established order within an annealing furnace, and annealing the steel sheet; a step of applying a hot-dip galvanized coating onto the steel sheet discharged from the cooling zone; and a step of thermally alloying the galvanizing, where a mixed gas of humidified gas and dry gas and a dry gas are supplied to the homogenization zone; a dry gas LJ IQI η / 77P7 / B / YILI is supplied to the cooling zone; and the volume Vr of the homogenization zone, the gas flow rate Qrw and the water content Wr of the humidified gas supplied to the homogenization zone, the gas flow rate Qrd of the dry gas supplied to the homogenization zone, the gas flow rate Qcd of the dry gas supplied to the cooling zone, and the average temperature Tr within the homogenization zone satisfy a predetermined relationship. This is a technology where the surface of the steel sheet is sufficiently oxidized using the direct flame furnace in the heating zone, and then the dew point throughout the homogenization zone is set higher than the dew point of a conventional method to allow sufficient internal oxidation of the Si. In this way, the surface concentration of Si is suppressed and the alloying temperature is reduced.With this method, even when subjecting a steel sheet containing 0.2% by mass or more of Si to electro-annealing, it is possible to obtain high coating adhesion and a favorable coating appearance, and to suppress the decrease in breaking stress when lowering the alloy temperature. LIST OF APPOINTMENTS Patent Document PTL1:JP 2016-017192 A BRIEF DESCRIPTION OF THE INVENTION (Technical Problem) However, the method described in PTL 1 focuses solely on achieving a favorable coating appearance when applying a hot-dip galvanized coating to a high-tensile steel sheet with a Si content of 0.2% by mass or more, and does not address the case of continuously passing a steel sheet with a Si content of less than 0.2% by mass through the process (hereafter referred to as “a common steel sheet” in this specification). When the steel type changes, the desired annealing temperature (temperature on the outlet side of the homogenization zone) and the dew point of the homogenization zone also change. Therefore, it takes time to adjust the homogenization zone to an optimal low dew point for a common steel sheet with a Si content of less than 0.2%.2% by mass after passing a high tensile strength steel sheet having a Si content of 0.2% by mass or more and supplying a humidified gas to the entire homogenization zone to uniformly control the dew point across the entire homogenization zone to a high dew point, as described in PTL 1. As a result, oxidized metal adhesion defects occur on the annealed common steel sheet before the dew point change is completed (i.e., a tip portion of the steel sheet coil), and the tip portion must be cut off in subsequent processes, which reduces throughput. In view of this, the method described in PTL 1 still has room for improvement. It might thus be useful to provide a method for producing an annealed galvanized steel sheet and a continuous hot-dip galvanizing apparatus, with which high coating adhesion and a favorable coating appearance can be obtained when applying a hot-dip galvanized coating to a steel sheet having a Si content of 0.2% by mass or more, and at the same time, the dew point of the atmosphere in the homogenization zone can be changed rapidly so as to suppress the occurrence of oxidized metal adhesion defects when subsequently applying a hot-dip galvanized coating to a steel sheet having a Si content of less than 0.2% by mass. (Solution to the Problem) This disclosure aims to (A) suppress the concentration of silicon oxide on the surface of steel sheets and achieve good adhesion when passing through a high-tensile steel sheet with a silicon content of 0.2% by mass or more, and (B) suppress the occurrence of oxidized metal adhesion defects by rapidly changing the dew point of the atmosphere in the homogenization zone when a common steel sheet with a silicon content of less than 0.2% by mass is subsequently passed through simultaneously. According to our study, (A) can be achieved provided that a humidified gas is supplied specifically to a second part of the homogenization zone where the steel sheet reaches its highest temperature, and it is not necessary to supply a humidified gas to the entire homogenization zone to raise the dew point.When humidified gas is supplied only to the second part of the homogenization zone instead of the entire zone, the dew point within the homogenization zone can be rapidly lowered when passing through a type of steel that does not require any humidified gas supply, thus achieving (B). Additionally, according to our study, it is important to determine the interval within the second part of the homogenization zone where humidified gas should be supplied during the passage of a high-tensile steel sheet, taking into account a sheet through speed V and a target temperature T on the exit side of the homogenization zone. In this way, both (A) and (B) can be achieved. The main features of this disclosure, which was developed based on the previous conclusions, are as follows. [1] A method for producing an annealed galvanized steel sheet using a continuous hot-dip galvanizing apparatus comprising a vertical annealing furnace in which a heating zone, a homogenizing zone, and a cooling zone are arranged in the specified order, a hot-dip galvanizing apparatus located downstream of the cooling zone, and an alloying line located downstream of the hot-dip galvanizing apparatus, wherein the method comprises transferring a steel sheet within the annealing furnace through the heating zone, the homogenizing zone, and the cooling zone in the specified order, and performing annealing on the steel sheet, wherein the steel sheet is transferred vertically a plurality of times within each zone to form a plurality of passes,applying a hot-dip galvanized coating to the steel sheet discharged from the cooling zone using the hot-dip galvanizing apparatus, and thermally alloying the galvanized coating applied to the steel sheet using the alloying line, wherein a plurality of humidified gas supply orifices for supplying a reducing or non-oxidizing humidified gas to the homogenization zone, and at least one dry gas supply orifice for supplying a reducing or non-oxidizing dry gas to the homogenization zone are arranged in the, LJ IQI η / 77Π7 / Β / YILI homogenization zone, and in a case where the steel sheet being passed through the homogenization zone is of a type of steel containing 0.2% by mass or more of Si, both dry gas and humidified gas are supplied to the homogenization zone, wherein the humidified gas is supplied only from a humidified gas supply orifice located in a second part of the homogenization zone among the plurality of humidified gas supply orifices, wherein the second part of the homogenization zone is an area on the cooling zone side of a pass immediately upstream of a pass that includes a position further upstream of a portion of steel sheet corresponding to L, where L is determined in order to satisfy the following expression (1) 1.0<10100L / Vexp{-14560 / (T+273.15)}<2.5 (1) where L[m] is a length of steel sheet from one exit side of the homogenization zone, V[m / s] is a sheet passing speed, and T[°C] is a target temperature on the exit side of the homogenization zone. [2] The method for producing an annealed galvanized steel sheet in accordance with [1], wherein in the case where the steel sheet being passed through the homogenization zone is of a type of steel containing 0.2% by mass or more of Si, a furnace gas dew point taken from a dew point measuring hole located in the second part of the homogenization zone is controlled at -25 °C or higher and 0 °C or lower. [3] A continuous hot-dip galvanizing apparatus for carrying out the method for producing an annealed steel sheet in accordance with [1] or [2], comprising an annealing furnace in which a heating zone, a homogenizing zone, and a cooling zone are arranged in the specified order, a hot-dip galvanizing apparatus located downstream of the cooling zone, an alloying line located downstream of the hot-dip galvanizing apparatus, and a plurality of humidified gas supply orifices for supplying a reducing or non-oxidizing humidifying gas to the homogenizing zone, and at least one dry gas supply orifice for supplying a reducing or non-oxidizing dry gas to the homogenizing zone arranged in the homogenizing zone,wherein each of the plurality of humidified gas supply orifices has a regulating valve capable of independently controlling the supply and interruption of the humidified gas and a gas flow rate. (Advantageous Effect) In accordance with the method for producing an annealed galvanized steel sheet and the continuous hot-dip galvanizing apparatus of this disclosure, it is possible to obtain high coating adhesion and a favorable coating appearance when applying a hot-dip galvanized coating to a steel sheet having a Si content of 0.2% by mass or more, and at the same time, suppress the occurrence of oxidized metal adhesion defects by rapidly changing the dew point of the atmosphere in the homogenization zone when subsequently applying a hot-dip galvanized coating to a steel sheet having a Si content of less than 0.2% by mass. BRIEF DESCRIPTION OF THE FIGURES In the attached drawings: FIG. 1 is a schematic view illustrating the structure of a continuous hot-dip galvanizing apparatus 100 used in one embodiment of this disclosure; and FIG. 2 is a schematic view illustrating a supply system of a humidified gas and a dry gas to the homogenization zone 12 of FIG. 1. DETAILED DESCRIPTION OF THE INVENTION The structure of a continuous hot-dip galvanizing apparatus 100 used in a method for producing a galvano-annealed steel sheet in accordance with an embodiment of this disclosure is first described, with reference to FIG. 1. The continuous hot-dip galvanizing apparatus 100 includes a vertical annealing furnace 20 in which a heating zone 10, a homogenizing zone 12, and cooling zones 14 and 16 are arranged in the stated order, a hot-dip galvanizing bath 22 located downstream of the cooling zone 16 in the steel sheet passing direction, and an alloying line 23 located downstream of the hot-dip galvanizing bath 22 in the steel sheet passing direction.The cooling zone for this method includes a first cooling zone 14 (rapid cooling zone) and a second cooling zone 16 (slow cooling zone). A nozzle 18 connected to the second cooling zone 16 has its tip submerged in the hot-dip galvanizing bath 22, thereby connecting the annealing furnace 20 and the hot-dip galvanizing bath 22. A steel sheet P is fed into the heating zone 10 through a steel sheet feed opening at the bottom of the heating zone 10. One or more hearth rolls are arranged at the top and bottom of each of zones 10, 12, 14, and 16. In the case where the steel sheet P is bent downwards by 180 degrees on one or more hearth rolls, the steel sheet P is transferred vertically a plurality of times within the corresponding predetermined zone of the annealing furnace 20, forming a plurality of passes. Figure 1 illustrates an example of having two passes in the heating zone 10, ten passes in the homogenizing zone 12, two passes in the first cooling zone 14, and two passes in the second cooling zone 16. However, the number of passes is not limited to these and may be set as appropriate according to the processing conditions.In some hearth rolls, the steel sheet P is not bent downwards but changes direction at a right angle to move to the next zone. The steel sheet P is annealed in this manner within the annealing furnace 20 as it is transferred through the heating zone 10, the homogenizing zone 12, and the cooling zones 14 and 16 in the specified order. Each of zones 10, 12, 14, and 16 is a vertical kiln, where the height is not particularly limited and can be approximately 20 to 40 m. Additionally, the length of each zone (horizontal direction in FIG. 1) can be determined as appropriate according to the number of passes in each zone. For example, the heating zone 10 with two passes can be approximately 0.8 to 2 m, the homogenization zone 12 with ten passes can be approximately 10 to 20 m, and the first cooling zone 14 and the second cooling zone 16, each with two passes, can each be approximately 0.8 to 2 m. Adjacent zones in the annealing furnace 20 communicate via a connecting portion that links the upper or lower portions of the respective zones. In this configuration, the heating zone 10 and the homogenizing zone 12 communicate via a throat (restriction portion) that links the lower portions of the respective zones. The homogenizing zone 12 and the first cooling zone 14 communicate via a throat that links the lower portions of the respective zones. The first cooling zone 14 and the second cooling zone 16 communicate via a throat that links the lower portions of the respective zones. The height of each throat can be set as appropriate. However, to maximize the atmospheric independence of each zone, it is preferable for the height of each throat to be as low as possible.The gas in the annealing furnace 20 flows from downstream to upstream in the furnace, and is discharged from the steel sheet introduction port at the bottom of the heating zone 10. (Warming area) In this configuration, heating zone 10 is capable of indirectly heating the steel sheet P using a radiant tube (RT) or an electric heater. The average temperature within heating zone 10 is preferably between 700 °C and 900 °C. In heating zone 10, a reducing gas or a non-oxidizing gas is supplied separately, while a gas flows in from the homogenization zone 12. A mixed H₂-N₂ gas is typically used as the reducing gas. Examples of the reducing gas include a gas (dew point: approximately -60 °C) with a composition of 1% to 20% H₂ by volume, the remainder being N₂ and unavoidable impurities. Similarly, examples of the non-oxidizing gas include a gas (dew point: approximately -60 °C) with a composition of N₂ and unavoidable impurities. The gas supply to heating zone 10 is not particularly restricted.However, the gas is preferably supplied from two or more vertical inlet ports and one or more longitudinal inlet ports to ensure uniform distribution within the heating zone. The gas flow rate supplied to the heating zone is measured by a gas flow meter (not shown) installed in a pipe. The gas flow rate is not particularly limited and can range from approximately 10 to 100 Nm³ / h. (Homogenization Zone) In this configuration, the homogenization zone 12 is capable of indirectly heating the steel sheet P using a radiant (unpolished) tube as a heating medium. The average temperature within the homogenization zone 12 is preferably between 700 °C and 1000 °C. A reducing gas or a non-oxidizing gas is supplied to the homogenization zone 12. A mixed H2-N2 gas is typically used as the reducing gas. Examples of the reducing gas include a gas (dew point: approximately -60 °C) with a composition of 1% to 20% H2 by volume, the remainder being N2 and unavoidable impurities. Additionally, examples of the non-oxidizing gas include a gas (dew point: approximately -60 °C) with a composition of N2 and unavoidable impurities. In this mode, the reducing gas or non-oxidizing gas supplied to the homogenization zone 12 has two forms, namely humidified gas and dry gas. Here, “dry gas” is the reducing gas or non-oxidizing gas that has a dew point of approximately -60 °C to -50 °C and is not humidified by a humidifying device, and “humidified gas” is the gas that has been humidified to a dew point of 0 °C to 30 °C by a humidifying device. Figure 2 is a schematic view illustrating humidified gas and dry gas supply systems to the homogenization zone 12. The humidified gas is supplied by means of three systems, namely, humidified gas supply ports 44A to 44E, humidified gas supply ports 45A to 45E, and humidified gas supply ports 46A to 46E. In Figure 2, the reducing gas or non-oxidizing gas (dry gas) is partially sent to a humidifying device 26 by means of a dry gas distribution device 24, and the remainder passes through a dry gas pipe 30 as dry gas and is supplied to the homogenization zone 12 by means of dry gas supply ports 32A, 32B, 32C, and 32D. The position and number of dry gas supply orifices are not particularly limited and can be determined as appropriate, taking various conditions into consideration. However, it is preferable that a plurality of dry gas supply orifices be arranged at the same height along the longitudinal direction of the homogenization zone, and that the dry gas supply orifices be evenly spaced along the longitudinal direction of the homogenization zone. The gas humidified by the humidifying device 26 passes through a humidified gas pipe 40 and is distributed to the three systems by means of a humidified gas distribution device 39 and is supplied to the homogenization zone 12 by means of the humidified gas supply orifices 44A to 44E, the humidified gas supply orifices 45A to 45E, and the humidified gas supply orifices 46A to 46E through a humidified gas pipe 43 respectively. The position and number of humidified gas supply orifices are not particularly limited and can be determined as appropriate, taking various conditions into consideration. However, it is preferable that a plurality of humidified gas supply orifices be arranged at the same height along the longitudinal direction of the homogenization zone, and that the humidified gas supply orifices be evenly spaced along this direction. Additionally, it is preferable that one or more rows of humidified gas supply orifices be provided along the longitudinal direction of the homogenization zone in each of two vertically divided areas of the homogenization zone 12. In this way, the dew point can be controlled uniformly throughout the homogenization zone 12.Reference number 41 is a humidified gas flow meter, and reference number 42 is a humidified gas dew point meter. The humidifier device 26 includes a humidification module that has a hollow fiber membrane, either fluorine-based or polyimide-based, a flat membrane, or a similar material. Dry gas flows into the membrane, while pure water, adjusted to a predetermined temperature in a constant-temperature circulating water bath 28, circulates outside the membrane. The hollow fiber membrane, whether fluorine-based or polyimide-based, or the flat membrane, is a type of ion-exchange membrane that has an affinity for water molecules. When a difference in water concentration occurs between the inside and outside of the hollow fiber membrane, a force is generated to compensate for the concentration difference. This force acts as a driving force, causing water to move through the membrane toward the area with the lower water concentration. The temperature of the dry gas changes as the air temperature changes seasonally or daily.In this humidifying device, however, heat exchange is achieved by ensuring sufficient contact area between the gas and water through a vapor-permeable membrane. Consequently, regardless of whether the dry gas temperature is higher or lower than the circulating water temperature, the dry gas is humidified to the same dew point as the set water temperature, thus achieving highly precise dew point control. The dew point of the humidified gas can be controlled to any value within the range of 5°C to 50°C. If the dew point of the humidified gas is higher than the pipe temperature, condensation occurs in the pipe, and the condensed water can infiltrate directly into the furnace. Therefore, the humidified gas pipe is heated and maintained at a temperature no lower than the dew point of the humidified gas and no lower than the outside air temperature. Here, in the case of producing a high tensile strength steel sheet with a chemical composition containing 0.2% or more Si by mass, humidified gas is supplied to homogenization zone 12 in addition to dry gas to raise the dew point within the homogenization zone. On the other hand, in the case of producing a steel sheet with a Si content of less than 0.2% by mass (for example, a common steel sheet with a tensile strength of approximately 270 MPa), only dry gas is supplied to homogenization zone 12, and no mixed gas is supplied. This modality is characterized in that, when a high-tensile steel sheet with a silicon content of 0.2% by mass or more is passed through, the humidified gas is supplied only from a second part of the homogenization zone where the steel sheet reaches a higher temperature. The interval of this second part of the homogenization zone is determined by considering a sheet passing speed V and a target temperature T on the exit side of the homogenization zone. The technical importance of adopting such a characteristic structure is described below. In order to allow such control of the humidified gas supply, all the humidified gas supply ports in this modality are independent of each other and have a regulating valve 50 that can control the supply and interruption of the humidified gas and the gas flow rate, as illustrated in FIG. 2. The temperature of the steel sheet on the outlet side of the heating zone is set approximately 300°C to 500°C lower than the steel sheet temperature (annealing temperature) on the outlet side of the homogenization zone. For example, when the steel sheet temperature on the outlet side of the homogenization zone is 850°C, the steel sheet temperature on the outlet side of the heating zone is approximately 350°C to 550°C, and the steel sheet is heated to 300°C to 500°C in the initial part of the homogenization zone. Furthermore, the silicon added to the steel is more concentrated on the surface of the steel sheet as the temperature exceeds 700°C.It was found that, in order to suppress surface concentration, the dew point in the second part of the homogenization zone, where the steel sheet reaches a higher temperature, can be set between -25 °C and 0 °C, and that silicon promotes oxide formation within the steel sheet and has the effect of improving coating adhesion and promoting the alloying reaction. Additionally, it was found that the interval within the second part of the homogenization zone where the humidified gas should be supplied can be determined based on the following expression (1). 1.0<10100L / Vexp{-14560 / (T+273.15)}<2.5 (1) L[m]: length of steel sheet from the outlet side of the homogenization zone V[m / sj: sheet passage speed T[°C]: Target temperature on the outlet side of the homogenization zone Here, the sheet passage speed V and the target temperature T on the exit side of the homogenization zone are predetermined when passing a high tensile strength steel sheet with a Si content of 0.2% by mass or more. Typically, the sheet passage speed V is determined from the range of 1.0 m / s to 2.0 m / s, taking into account, for example, the thickness of the steel sheet, and the target temperature T on the exit side of the homogenization zone is determined from the range of 750 °C to 900 °C, taking into account, for example, the chemical composition of the steel sheet.The “target temperature on the outlet side of the homogenization zone” is a target temperature of the steel sheet on the outlet side of the homogenization zone established based on the material control of the steel sheet, and the temperature within the homogenization zone is controlled so that the steel sheet temperature measured by a radiation thermometer reaches the target temperature. Next, the sheet pass speed V and the target temperature T on the exit side of the homogenization zone, which are determined in advance, are substituted into expression (1), and the length of steel sheet L from the exit side of the homogenization zone is determined to satisfy expression (1). The length of steel sheet L from the exit side of the homogenization zone is the length of steel sheet from a bottom flat roll 49E on the exit side of the homogenization zone that is located most downstream among the bottom flat rolls 49 of the homogenization zone, with reference to FIG. 2. Additionally, the area on the cooling zone side of the pass immediately upstream of the pass corresponding to the most upstream position of the portion of steel sheet corresponding to the determined L is defined as the second part of the homogenization zone.Referring to FIG. 2, Pi indicates the upstream position of the steel sheet portion of length L from the exit side of the homogenization zone. The area on the cooling zone side, namely the downstream side in the longitudinal direction of the homogenization zone, of the pass (the fourth pass in FIG. 2) immediately upstream of the pass (the fifth pass in FIG. 2) corresponding to the upstream position Pi is the second part 12B of the homogenization zone. The area on the heating zone side, namely the upstream side in the longitudinal direction of the homogenization zone, of the pass (the fourth pass in FIG. 2) immediately upstream of the pass corresponding to the upstream position Pi is the first part 12A of the homogenization zone.In this mode, the humidified gas is supplied only from the humidified gas supply ports located in the second part 12B of the homogenization zone (in FIG. 2, humidified gas supply ports 44C to 44E at the top, humidified gas supply ports 45C to 45E in the middle, and humidified gas supply ports 46C to 46E at the bottom) among the plurality of humidified gas supply ports. In this way, (A) when a high tensile strength steel sheet having a Si content of 0.2% by mass or more is passed through, it is possible to suppress the concentration of Si oxides on the surface of the steel sheet and achieve good adhesion, and (B) when a common steel sheet having a Si content of less than 0.2% by mass is passed immediately after the high tensile strength steel sheet. This makes it possible to suppress the appearance of oxidized metal adhesion defects by rapidly changing the dew point of the atmosphere in the homogenization zone. In accordance with the definition of the second part of the homogenization zone, humidified gas is supplied to the front and back of the steel sheet in the pass corresponding to the upstream position Pi of the steel sheet portion of length L from the exit side of the homogenization zone. Setting the value of the second part of expression (1) to 1.0 or higher is a necessary condition to ensure the minimum required internal oxidation of Si. Therefore, if the value of the second part is less than 1.0, insufficient internal oxidation of Si occurs when passing a high tensile strength steel sheet with a Si content of 0.2% by mass or higher, making it impossible to achieve high coating adhesion or a favorable coating appearance. Furthermore, the alloying temperature increases and the tensile strength decreases. Therefore, in this scenario, the value of the second part is 1.0 or higher. Furthermore, establishing the dew point at 2.5 or less is a necessary condition for rapidly changing the atmosphere in the homogenization zone. Therefore, if the dew point exceeds 2.5, it takes time to change the dew point when a high-tensile steel sheet with added silicon is converted to a common steel sheet, and surface defects such as oxidized metal adhesion defects occur during the production of the common steel sheet. Additionally, the effect of improving coating adhesion and promoting the alloying reaction is saturated even if the dew point exceeds 2.5 and the humidification area is extended. Therefore, in this mode, the dew point is set at 2.5 or less. For example, the following can be established in the actual operation. For example, in the case of a sheet feed speed V = 2.0 m / s, if the target temperature T on the exit side of the homogenization zone = 750 °C, then the length of steel sheet from the exit side of the homogenization zone that satisfies expression (1) is 301 m < L < 750 m; if the target temperature T on the exit side of the homogenization zone = 800 °C, then the length of steel sheet from the exit side of the homogenization zone that satisfies expression (1) is 155 m < L < 387 m. In the case where it is required that the sheet feed speed be maintained at 2.0 m / s during the operation, the second part of the homogenization zone is set with L = 301 m, for example, in order to satisfy 301 m < L < 387 m.In this way, it is possible to carry out an operation that satisfies expression (1) regardless of whether the target temperature T on the outlet side of the homogenization zone is 750 °C or 800 °C, so that it is not necessary to make major changes to the operating conditions other than changing the target temperature T. Additionally, in the case of a sheet feed speed V = 1.0 m / s, if the target temperature T on the exit side of the homogenization zone = 750 °C, the length of steel sheet from the exit side of the homogenization zone that satisfies expression (1) is 151 m < L < 375 m. Therefore, after carrying out an operation where the sheet feed speed = 2.0 m / s and the target temperature T on the exit side of the homogenization zone = 800 °C (an operation where L that satisfies expression (1) is in the range of 155 m to 387 m), if it is desired to carry out an operation where the target temperature T on the exit side of the homogenization zone is changed to 750 °C, L can be fixed at 155 m more by setting the sheet feed speed to 1.0 m / s. In other words, there is no need to expand the second part of the homogenization zone, which is preferable from the point of view of accelerating the atmospheric change. The flow rate of the humidified gas supplied to homogenization zone 12 is not particularly limited, provided it is controlled as described above. However, it is maintained in the range of approximately 100 to 400 Nm³ / h. The flow rate of the dry gas supplied to homogenization zone 12 is not particularly limited. However, it is maintained in the range of approximately 10 to 300 Nm³ / h when passing through a high-tensile steel sheet with a chemical composition containing 0.2% by mass or more Si, and in the range of 200 to 600 Nm³ / h when passing through a steel sheet with a Si content of less than 0.2% by mass (e.g., a common steel sheet with a tensile strength of approximately 270 MPa). (Cooling Zone) In this mode, the steel sheet P is cooled in cooling zones 14 and 16. The steel plate P is cooled to approximately 480 °C to 530 °C in the first cooling zone 14 and to approximately 470 °C to 500 °C in the second cooling zone 16. Cooling zones 14 and 16 are also supplied with a reducing or non-oxidizing gas, and only dry gas is supplied. The supply of dry gas to cooling zones 14 and 16 is not particularly limited. However, the dry gas is preferably supplied from two or more inlet ports vertically and two or more inlet ports longitudinally to ensure uniform distribution within the cooling zones. The total flow rate of dry gas supplied to cooling zones 14 and 16 is measured by a gas flow meter (not shown) installed in a pipeline. The total flow rate is not particularly limited and can range from approximately 200 to 1000 Nm³ / h. (Hot-dip galvanizing bath) The hot-dip galvanizing bath 22 can be used to apply a hot-dip galvanized coating onto the steel sheet P discharged from the second cooling zone 16. The hot-dip galvanized coating can be applied using a conventional method. (Alloy line) The 23 alloy line can be used for thermally alloying the galvanizing applied to steel sheet P. The alloying treatment can be carried out using a conventional method. In this method, the alloying temperature is kept low to minimize the reduction in tensile strength of the resulting electro-annealed steel sheet. (Chemical composition of steel sheet) The steel sheet P that will be subjected to annealing and hot-dip galvanizing treatment in The hot LJ IQI η / 77η7 / B / YILI is not particularly limited. However, in the case of a steel sheet having a chemical composition containing 0.2% by mass or more of Si, namely high tensile strength steel, the effects of this disclosure can be advantageously obtained. A suitable chemical composition of the steel sheet is described below. In the following description, all units indicated by “%” refer to “% by mass”. The carbon content is preferably 0.025% or higher because carbon facilitates improved processability by forming a retained austenite layer or a martensitic phase in the steel's microstructure. However, this specification does not define a specific lower limit. Furthermore, if the carbon content exceeds 0.3%, weldability deteriorates. Therefore, the carbon content is preferably 0.3% or lower. Silicon (Si) is an effective element for strengthening steel and achieving good material properties, and therefore 0.2% or more Si is added to a high-tensile steel sheet. If the Si content is less than 0.2%, expensive alloying elements are required to obtain high strength. On the other hand, if the Si content is greater than 2.5%, the formation of an oxide coating during oxidation treatment is suppressed. Additionally, the alloying temperature increases, making it difficult to obtain the desired mechanical properties. Therefore, the Si content is preferably 2.5% or less. Manganese (Mn) is an effective element for reinforcing steel. The Mn content is preferably 0.5% or more to ensure a tensile strength of 590 MPa or higher. However, if the Mn content exceeds 3.0%, it can be difficult to guarantee weldability, coating adhesion, and a balance between strength and ductility. Therefore, the Mn content is preferably between 0.5% and 3.0%. For tensile strengths of 270 MPa to 440 MPa, Mn is appropriately added at a content of 1.5% or less. Phosphorus (P) is an effective element for strengthening steel. However, in the case of steel with a silicon (Si) content of 0.2% or more, the P content is preferably 0.03% or less because it slows down the alloying reaction between zinc and steel. It can be added appropriately to maintain strength in other cases. Although sulfur has little influence on the strength of steel, it affects the formation of oxide coatings during hot and cold rolling. Therefore, the sulfur content is preferably 0.005% or less. In addition to the elements mentioned above, one or more elements such as Cr, Mo, Ti, Nb, V, and B may be added optionally, for example. The remainder consists of Fe and unavoidable impurities. EXAMPLES (Experimental conditions) The continuous hot-dip galvanizing apparatus illustrated in Figures 1 and 2 was used to anneale four types of steel sheets, whose chemical composition was as indicated in Table 1, under various annealing conditions, and then subject the steel sheets to hot-dip galvanizing and alloying treatment. Steels B and C are high-tensile steels, and steels A and D are common steels. As indicated in Table 2, steels A, B, C, and D were passed continuously in the order given in Examples Nos. 1 to 4. The sheet passing rate is indicated in Table 1. LJ LQLn / ZZnZ / E / YIAI The heating zone was a RT furnace with a volume of 200 m³. The average temperature within the heating zone was 700 °C to 800 °C. In the heating zone, a gas (dew point: -50 °C) with a composition of 15% H₂ by volume, the remainder being N₂ and unavoidable impurities, was used as a dry gas. The flow rate of the dry gas to the heating zone was 100 Nm³ / h. The homogenization zone was a 700 m³ RT furnace. A gas (dew point: -50 °C) with a composition of 15% H₂ by volume, the remainder being N₂ and unavoidable impurities, was used as a dry gas. Part of the dry gas was humidified using a humidifying device with a hollow fiber membrane humidification section to produce a humidified gas. The hollow fiber membrane humidification section consisted of 10 membrane modules, and each module was allowed to flow at a maximum of 500 L / min of dry gas and a maximum of 20 L / min of circulating water. A common constant-temperature circulating water bath capable of supplying a total of 200 L / min of pure water was used. Dry gas supply holes and humidified gas supply holes were arranged in the positions illustrated in FIG. 2. That is, humidified gas introduction holes were provided in five locations along the longitudinal direction of the homogenization zone in each of the upper, middle, and lower parts of the homogenization zone corresponding to the arrangement of hearth rollers in the oven (five upper hearth rollers and five lower hearth rollers), that is, humidified gas introduction holes were provided in a total of 15 locations in five rows (three locations per row) in the vertical direction of the homogenization zone, and each humidified gas supply hole was provided with an on-off valve so that each humidified gas supply hole could independently control the supply of humidified gas.The length between the upper and lower floor rollers of the homogenization zone was 30 m, and a row of humidified gas introduction holes functioned as the humidification area for a length of steel sheet of 60 m (2 passes). The target temperature on the outlet side of the homogenization zone and the target dew point within the homogenization zone during the passage of steels A through D are indicated in Table 1. Additionally, dry gas was supplied at the flow rate indicated in Table 2 to the homogenization zone during the passage of each steel. Furthermore, humidified gas was supplied only from the humidified gas supply orifice located in a second part of the homogenization zone determined based on L, as indicated in Table 2, and the total flow rate was as indicated in Table 2. The “number of humidified gas introduction rows” in Table 2 indicates the number of rows of humidified gas supply orifices in the second part of the homogenization zone among the five rows along the vertical direction of the homogenization zone. As illustrated in FIG.2. With respect to the positions of the humidified gas supply orifices, the upper humidified gas supply orifices 44A to 44E and the lower humidified gas supply orifices 46A to 46E were arranged in the same position along the longitudinal direction of the homogenization zone, while the middle humidified gas supply orifices 45A to 45E were arranged half a step apart along the longitudinal direction of the homogenization zone. This allowed for uniform humidification of the steel sheet surface. Note that 44A, 45A, and 46A are considered as one. LJ IQI Π / ΖΖΠΖ / Β / ΥΙΛΙ row to count the number of introduction rows. The same applies to symbols B to E. The “first part dew point” and “second part dew point” columns for the homogenization zone in Table 2 indicate the dew point in the homogenization zone measured at the dew point measurement orifices 47A and 47B, respectively, in Figure 2. The “steel sheet temperature measured at the outlet side” in Table 2 is the steel sheet temperature measured at the outlet side of the homogenization zone. Additionally, the “humidified gas dew point” indicates the dew point measured by the humidified gas dew point meter 42 in Figure 2. Dry gas (dew point: -50 °C) was supplied to the first cooling zone and the second cooling zone from the bottom of each zone at the flow rate indicated in Table 2. The galvanizing bath temperature was 460 °C, the Al concentration in the galvanizing bath was 0.130%, and the coating weight was adjusted to 50 g / m² per side by gas purging. After hot-dip galvanizing, alloying treatment was carried out in an induction-heated alloying furnace so that the coating alloy grade (Fe content) was 10% to 13%. The alloying temperature during the treatment is shown in Table 2. (Evaluation method) The coating appearance was evaluated through inspection using an optical surface defect meter (detecting coating loss or oxidized metal adhesion defects on rollers with a diameter of φ 0.5 or greater) and visual determination of alloy irregularity. Those that met all criteria were considered “good,” those with a low degree of alloy irregularity were considered “fair,” and those rejected for at least one criterion were considered “poor.” The results are shown in Table 2. Additionally, the tensile strength of the galvano-annealed steel sheets produced under various conditions was measured. Steel A was considered acceptable when its tensile strength was 270 MPa or higher, steel B was 780 MPa or higher, steel C was 980 MPa or higher, and steel D was 340 MPa or higher. The results are shown in Table 2. Table 1 (% by mass) Maple Chemical composition Target temperature in homogeneity zone C Si Mn PS Salt side temperature (°C) Rock point of first part (°C) Rock point of second part (°C) A 0.08 0.01 0.2 0.02 0.001 740 ± 20 -50 to -30 -50 to -30 B 0.10 0.2 2.4 0.02 0.001 800 ±15 -25 to-15 -15a -5 C 0.11 1.5 2.7 0.01 0.001 830 ±15 -20 to-10 -10 to 0 D 0.08 0.03 0.4 0.04 0.001 780 ± 20 -50 to -30 -50 to -30 Remainder: Fe and impurezas inevitables ω ω μ ιό σι ο σι ο σι ο σι Table 2 No. Steel Sheet width Sheet pass speed V (m / s) Homogenization zone First part dew point (°C) Second part dew point (°C) Target steel sheet temperature at outlet side T Π Measured steel sheet temperature at outlet side (°C) Dry gas flow rate (Nm3 / h) Humidified gas flow rate (Nm3 / h) Humidified gas dew point (°C) 1 A 1.2 1.6 38.2 36.5 740 745 440 0 - B 1.0 1.6 45.2 46.3 800 802 430 0 C 1.0 1.6 47.1 49.5 830 833 435 0 D 1.5 1.6 40.7 41.0 780 785 420 0 2 A 1.2 1.6 39.1 38.3 740 741 435 0 B 1.0 1.6 20.3 10.5 800 800 270 150 19 C 1.0 1.6 12.2 8.2 830 830 100 320 19 D 1.5 1.6 34.9 32.1 780 780 430 0 - 3 A 1.2 1.2 36.3 37.5 740 741 435 0 - B 1.0 1.2 22.2 13.2 800 804 270 80 19 C 1.0 1.2 13.5 8.2 830 840 160 260 19 D 1.5 1.2 32.2 33.4 780 779 430 0 4 A 1.2 1.2 36.3 37.5 740 741 435 0 B 1.0 1.2 24.1 13.2 800 802 270 80 19 C 1.0 1.5 12.9 7.2 830 840 100 320 19 D 1.5 1.5 26.6 24.3 780 779 430 0 > a i\ C hhc ó ω ω ιυ μ σι o σι o σι o σι Continuation of table 2 No. Humidification Zone Cooling Zone Alloy Treatment Coating Appearance Material Strength Category Number of Rows of Humidified Gas Introduction Humidification Area L (m) Second part of expression (1) Expression (1) Gas flow rate (Nm3 / h) Alloy temperature ΓO Tensile strength (IMPa) Pass 0 Failed 1 0 0 0.00 550 495 Good 298 Pass Comparative Example 0 0 0.00 Not Satisfied 550 550 Bad 752 Failed 0 0 0.00 Not Satisfied 550 562 Bad 962 Failed 0 0 0.00 550 490 Good 368 Pass 2 0 0 0.00 550 495 Good 302 Pass Example 3 180 1.46 Satisfied 550 510 Good 802 Approved 3 180 2.11 Satisfied 550 503 Good 1010 Approved 0 0 0.00 550 490 Good 360 Approved 3 0 0 0.00 550 495 Good 292 Approved Example 2 120 1.29 Satisfied 550 515 Good 799 Approved 2 120 1.87 Satisfied 550 505 Good 1008 Approved 0 0 0.00 550 495 Good 365 Approved 4 0 0 0.00 550 495 Good 301 Approved Comparative Example 1 60 0.65 Not satisfied 550 570 Bad 735 Failed 4 240 2.99 Not satisfied 550 505 Good 1012 Passed 0 0 0.00 550 492 Bad 361 Passed. > Q l\ chhc ó (Evaluation results) In the case of No. 1, when high-tensile-strength steels B and C with added Si were passed, no humidifying gas was supplied, and the value of the second part of expression (1) was 0. Therefore, the internal oxidation of Si was insufficient, and a favorable coating appearance could not be obtained. Additionally, the alloying temperature increased, and the tensile strength decreased. In the case of No. 4, when high-tensile-strength steel B with added Si was passed, the value of the second part of expression (1) was 0.65. Therefore, the internal oxidation of Si was insufficient, and a favorable coating appearance could not be obtained. Additionally, the alloying temperature increased, and the tensile strength decreased. Furthermore, when high-tensile-strength steel C with added Si was passed, the value of the second part of expression (1) was 2.99.Therefore, although the coating appearance of steel C was favorable, surface defects such as oxidized metal adhesion defects occurred on steel D subsequently, and the coating appearance of the same was affected because it took time to change the dew point. Furthermore, in cases Nos. 2 and 3, humidified gas was supplied to satisfy expression (1) during the passage of high tensile strength steels B and C with added Si. Therefore, steels B and C achieved a favorable coating appearance, and the subsequently passed steel D also achieved a favorable coating appearance. INDUSTRIAL APPLICATION In accordance with the method for producing an annealed galvanized steel sheet and the continuous hot-dip galvanizing apparatus of this disclosure, it is possible to obtain high coating adhesion and a favorable coating appearance when applying a hot-dip galvanized coating to a steel sheet having a Si content of 0.2% by mass or more, and at the same time, suppress the occurrence of oxidized metal adhesion defects by rapidly changing the dew point of the atmosphere in the homogenization zone when subsequently applying a hot-dip galvanized coating to a steel sheet having a Si content of less than 0.2% by mass. LIST OF REFERENCE SIGNS 100 continuous hot-dip galvanizing apparatus heating zone homogenization zone The first part of the homogenization zone 12B second part of the homogenization zone first cooling zone (rapid cooling zone) second cooling zone (slow cooling zone) tuyere annealing furnace hot-dip galvanizing bath alloy line dry gas distribution device humidifying device constant temperature circulating water bath dry gas pipe dry gas flow meter dry gas supply orifice humidified gas distribution device and 43 humidified gas pipe humidified gas flow meter humidified gas dew point meter 44A to 44E humidified gas supply port 45A to 45E humidified gas supply orifice 46A to 46E humidified gas supply port 47A and 47B dew point measuring hole upper sill roller lower sill roller 49E lower floor roller on the outlet side of the homogenization zone regulating valve P steel sheet Pi position most upstream of the steel sheet portion of length L from the outlet side of the homogenization zone

Claims

1. A continuous hot-dip galvanizing apparatus characterized in that it comprises: a vertical annealing furnace in which a heating zone, a homogenizing zone and a cooling zone are arranged in the specified order, one or more hearth rolls arranged in the vertical annealing furnace and which transport a steel sheet within the annealing furnace through the heating zone, the homogenizing zone and the cooling zone in the specified order to anneale the steel sheet, wherein the steel sheet is transferred vertically a plurality of times within each zone to form a plurality of passes, a hot-dip galvanizing apparatus positioned downstream of the cooling zone and which applies a hot-dip galvanized coating onto the steel sheet discharged from the cooling zone,an alloy line placed downstream of the hot-dip galvanizing apparatus and which thermally alloys the galvanized coating applied to the steel sheet, and a plurality of humidified gas supply orifices for supplying at least one of a reducing or non-oxidizing humidified gas to the homogenization zone, and at least one dry gas supply orifice for supplying a reducing or non-oxidizing dry gas to the homogenization zone arranged in the homogenization zone, wherein each of the plurality of humidified gas supply orifices has a regulating valve capable of independently controlling the supply and interruption of the humidified gas and a gas flow rate, when the steel sheet having a Si content of 0.2% by mass or more is conveyed into the annealing furnace,The plurality of humidified gas supply orifices supplies humidified gas to the homogenization zone and at least one dry gas supply orifice supplies dry gas to the homogenization zone, wherein the humidified gas is supplied only from a humidified gas supply orifice located in a second part of the homogenization zone between the plurality of humidified gas supply orifices, wherein the second part of the homogenization zone is an area on the cooling zone side of a pass immediately upstream of a pass that includes a more upstream position of a portion of steel sheet corresponding to L, where L is determined in order to satisfy the following expression (1) 1.0 < 10100 L / V exp {-14560 / (T+273.15)} <2.5 (1) where L[m] is a length of steel sheet from an outlet side of the homogenization zone,V[m / s] is a sheet passage speed in the range of 1.0 m / s to 2.0 m / s, and T[°C] is a target temperature on the exit side of the homogenization zone in the range of 750°C to 900°C, and when the steel sheet having a Si content of less than 0.2% by mass is conveyed into the annealing furnace, the plurality of the humidified gas supply holes do not supply humidified gas to the homogenization zone and at least one dry gas supply hole supplies dry gas to the homogenization zone.

2. The continuous hot-dip galvanizing apparatus according to claim 1, further characterized in that it additionally comprises a dew point measuring orifice located in the second part of the homogenization zone that collects the furnace gas, wherein when the steel sheet having a Si content of 0.2% by mass or more is conveyed into the annealing furnace, the dew point of the furnace gas collected from the dew point measuring orifice is controlled from -25 °C or higher and 0 °C or lower.