Continuous annealing equipment for steel sheets, continuous annealing method for steel sheets, and manufacturing method for galvanized steel sheets

Abstract

Provided are continuous annealing equipment for steel sheets, a continuous annealing method for steel sheets, and a method for manufacturing galvanized steel sheets, which are capable of manufacturing steel sheets with targeted mechanical properties by rapidly heating the steel sheets almost uniformly across the width of the steel sheets. [Solution] A continuous annealing equipment (1) for steel sheet (S) in which a heating zone (12), a soaking zone (14), a rapid heating zone (15), and a cooling zone (16) are arranged in this order in the conveying direction of the steel sheet (S), and the equipment has a plating device (5) that applies iron-based plating to the steel sheet (S) upstream of the heating zone (12) in the conveying direction, and a solenoid-type induction heating device (15A) provided in the rapid heating zone (15).

Description

[Technical Field]

[0001] The present invention relates to a continuous annealing facility for steel sheets, a continuous annealing method for steel sheets, and a method for producing galvanized steel sheets. [Background technology]

[0002] To reduce the weight and improve the safety of automobiles, there is a demand for higher strength galvanized steel sheets (hereinafter simply referred to as steel sheets). For example, during annealing, the steel sheet is heated above the ferrite / austenite two-phase region temperature, and then the austenite phase is transformed into a hard structure such as martensite or bainite during subsequent cooling. This process produces steel sheets with the desired mechanical properties. Radiant tube heating devices (sometimes also referred to as radiation heating devices) are commonly used for heating in annealing furnaces. However, radiant tube heating devices have a slow heating rate, and the size of the heating device and annealing furnace must be large to ensure sufficient annealing time. Furthermore, such heating devices have poor controllability of the annealing temperature of the steel sheet. As a result, when radiant tube heating devices are used as heating devices in annealing furnaces, it is difficult to precisely control the annealing temperature of the steel sheet in the longitudinal direction, which may make it difficult to control the material properties of the steel sheet.

[0003] To address the above-mentioned problems, Patent Document 1 discloses a technique in which, in continuous annealing equipment, a steel sheet is slowly heated to a temperature of 650° C. to 750° C. and then rapidly heated from there to a maximum annealing temperature of 750° C. to 910° C. The technique in Patent Document 1 is targeted at IF steel (interstitial free steel), and the maximum annealing temperature is controlled by rapid heating using an induction heating device or an electric current heating device.

[0004] Patent Document 2 discloses a technology for controlling the annealing temperature of a steel sheet with high precision by installing a transverse induction heating device on the outlet side of the soaking zone of a continuous annealing facility. The transverse induction heating device can heat the steel sheet regardless of the austenite fraction of the steel sheet, and can control the annealing temperature of the steel sheet in the longitudinal direction with high precision in the continuous annealing facility. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-144262 [Patent Document 2] Japanese Patent Application Publication No. 2024-048291 Summary of the Invention [Problem to be solved by the invention]

[0006] To produce a steel sheet with the desired mechanical properties, as described above, the steel sheet is heated to or above the ferrite / austenite two-phase region temperature, transforming the steel sheet's structure from ferrite to austenite. Then, during subsequent cooling, the austenite phase is transformed into a hard structure such as martensite or bainite. However, in the technology of Patent Document 1, the maximum annealing temperature is below the ferrite-to-austenite transformation point (910°C), so the structure of the IF steel of Patent Document 1 is primarily ferrite during annealing. Therefore, the technology of Patent Document 1 may not be able to produce a steel sheet with the desired mechanical properties. In other words, the technology of Patent Document 1 is a technology for heating a steel sheet with a low austenite fraction, and does not take into account the heating of a steel sheet with a high austenite fraction.

[0007] Examples of the induction heating device mentioned above include a transverse type induction heating device and a solenoid type induction heating device. Due to its structure, a transverse type induction heating device can rapidly heat even a steel sheet with a high austenite fraction, but it is difficult to heat the steel sheet almost uniformly in the width direction, which may cause variations in the mechanical properties of the steel sheet in the width direction. On the other hand, due to its structure, a solenoid type induction heating device can rapidly heat a steel sheet almost uniformly in the width direction, but it is difficult to heat a steel sheet with a high austenite fraction, which may cause the steel sheet to be unable to reach the target annealing temperature.

[0008] In the technology of Patent Document 2, a steel sheet is heated by a transverse induction heating device, so that the steel sheet can be heated regardless of the austenite fraction. However, due to its structure, it is difficult for a transverse induction heating device to heat the steel sheet almost uniformly in the width direction. Therefore, when a steel sheet is rapidly heated by a transverse induction heating device, temperature variations occur in the width direction of the steel sheet, which may cause variations in the mechanical properties of the steel sheet in the width direction.

[0009] The present invention has been made to solve the above-mentioned problems, and has an object to provide continuous annealing equipment for steel sheet, a continuous annealing method for steel sheet, and a method for manufacturing a galvanized steel sheet, which are capable of rapidly heating a steel sheet almost uniformly across the width direction of the steel sheet to produce a steel sheet having targeted mechanical properties. [Means for solving the problem]

[0010] The means for solving the above problems are as follows. [1] A continuous annealing facility for steel sheets, in which a heating zone, a soaking zone, a rapid heating zone, and a cooling zone are arranged in this order in the conveying direction of the steel sheets, the facility having a plating device that applies an iron-based plating to the steel sheets upstream of the heating zone in the conveying direction, and a solenoid-type induction heating device provided in the rapid heating zone. [2] A continuous annealing facility for steel sheets according to [1], further comprising a plating thickness measuring device for measuring the thickness of the iron-based plating on the steel sheets, and a control device for controlling at least one of the plating device and the solenoid-type induction heating device based on the thickness of the iron-based plating. [3] The continuous annealing equipment for steel sheets according to [2], further comprising a transformation rate meter that measures a phase fraction of the steel sheet, wherein the control device further controls the rapid heating zone based on the thickness of the iron-based coating and the phase fraction, and the transformation rate meter is provided between the rapid heating zone and the cooling zone in the conveying direction. [4] Continuous annealing equipment for steel sheets according to any one of [1] to [3], wherein the iron-based coating has a chemical composition containing, by mass%, 10% or less of at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder consisting of Fe and unavoidable impurities. [5] A continuous annealing facility for steel sheets according to any one of [1] to [3], further comprising a galvanizing zone for galvanizing the steel sheets downstream of the cooling zone in the conveying direction. [6] A continuous annealing method for steel sheet, which involves performing a heating step, a soaking step, a rapid heating step, and a cooling step on a steel sheet in this order, and which includes a plating step of applying an iron-based plating to the steel sheet using a plating device, and in the rapid heating step, heating the steel sheet that has been applied with the iron-based plating using a solenoid-type induction heating device. [7] A method for continuous annealing of steel sheet according to [6], wherein a thickness of the iron-based coating of the steel sheet is measured, and at least one of the plating device and the solenoid-type induction heating device is controlled based on the thickness of the iron-based coating. [8] A method for continuous annealing of a steel sheet according to [7], wherein a phase fraction of the steel sheet heated in the rapid heating step is measured, and the solenoid-type induction heating device is controlled based on the thickness of the iron-based coating and the phase fraction. [9] A method for producing a galvanized steel sheet, comprising: subjecting a steel sheet to continuous annealing by the method for continuous annealing of a steel sheet according to any one of [6] to [8]; and then galvanizing the steel sheet to produce a galvanized steel sheet. [Effects of the Invention]

[0011] According to the present invention, a steel sheet having the desired mechanical properties can be manufactured by rapidly heating the steel sheet almost uniformly across the width of the steel sheet. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram for explaining the configuration of a continuous annealing facility for steel sheets according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram for explaining the configuration of the control device shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] In a continuous annealing facility 1 for steel sheets according to an embodiment of the present invention (hereinafter referred to as the present embodiment), a rapid heating zone for rapidly heating a steel sheet to a target cooling start temperature is provided on the inlet side of a cooling zone in order to produce a steel sheet with target mechanical properties. A solenoid-type induction heating device is provided in the rapid heating zone as an induction heating device. To efficiently rapidly heat the steel sheet using the solenoid-type induction heating device, a plating device for applying an iron-based coating consisting of substantially pure iron, i.e., a ferrite phase, to the surface of the steel sheet is provided upstream of the rapid heating zone in the steel sheet transport direction. Therefore, by applying an iron-based coating to the surface of the steel sheet using the plating device, the rapid heating of the steel sheet using the solenoid-type induction heating device can be efficiently performed, and the steel sheet can be rapidly heated substantially uniformly in the width direction of the steel sheet. Furthermore, cooling the steel sheet in the cooling zone on the outlet side of the rapid heating zone can produce a hard structure in the steel sheet, thereby producing a steel sheet with target mechanical properties.

[0014] FIG. 1 is a diagram illustrating the configuration of continuous annealing equipment 1 for steel sheets according to this embodiment. As shown in FIG. 1, a steel sheet S wound into a coil in a previous process is unwound by a payoff reel 2 and enters a looper 4. In the example shown in FIG. 1, two payoff reels 2 are provided. The steel sheets S unwound from the payoff reels 2 are alternately transported to a welder 3 provided upstream of the looper 4 in the conveying direction of the steel sheet S (the left-right direction in FIG. 1). In the welder 3, the tail end of the preceding steel sheet S and the front end of the following steel sheet S are joined together by welding. The looper 4 is equipment that ensures an excess length of the steel sheet S so that the steel sheet S can continue to pass continuously when the steel sheets S are joined together by the welder 3.

[0015] (Iron-based plating equipment) An iron-based plating device 5 is provided between the welder 3 and the looper 4 in the conveying direction of the steel sheet S, applying an iron-based plating made of substantially pure iron to the surface of the steel sheet S. The iron-based plating device 5 is not limited to any particular type, and may be a conventionally known electrolytic or electroless plating device. Of these types, an electrolytic type is preferred, as it is capable of continuously applying an iron-based plating to the steel sheet S. Examples of electrolytic iron-based plating devices 5 include horizontal and roll types (not shown). The iron-based plating device 5 shown in FIG. 1 is a horizontal type. In this horizontal type iron-based plating device 5, a passage 9 defined by a pair of electrode plates 8 is formed between two pairs of rolls, i.e., a conductor roll 6 and a backup roll 7, in the conveying direction of the steel sheet S. Each electrode plate 8 extends along the conveying direction of the steel sheet S and is aligned in the height direction of the continuous annealing equipment 1 (the vertical direction in FIG. 1). The passage 9 is defined between the electrode plates 8 in the height direction. When the steel sheet S is passed through this passage 9, a plating solution is supplied to the gap between each electrode plate 8 and the steel sheet S, and an electric current is passed between the steel sheet S and each electrode plate 8, with each electrode plate 8 serving as an anode and the steel sheet S serving as a cathode, thereby applying an iron-based plating to the steel sheet S. The iron-based plating device 5 described above corresponds to the plating device of this embodiment.

[0016] A roll-type iron-based plating device has a plating bath that stores a plating solution and rolls that are placed in the plating bath, and the steel sheet S is wound around the rolls and immersed in the plating solution. In addition, electrode plates are placed along the steel sheet S on both sides of the rolls in the plating bath in the conveying direction of the steel sheet S. Each electrode plate serves as an anode, and the steel sheet S serves as a cathode, and an electric current is passed between the steel sheet S and each electrode plate to apply an iron-based plating to the steel sheet S.

[0017] Although the plating solution is not limited, a pure Fe-based plating solution is preferred. The pure Fe-based plating solution preferably contains, for example, 300 g / L of ferrous sulfate and 50 g / L of sodium sulfate, and has a hydrogen ion concentration (pH) of 1.8 to 2.2.

[0018] Incidentally, examples of electroless iron-based plating equipment include the conventionally known physical vapor deposition and chemical vapor deposition types.

[0019] The thickness of the iron-based coating formed on the surface of the steel sheet S by the iron-based coating device 5 is preferably 1 to 50 μm, and more preferably 10 to 30 μm. If the thickness of the iron-based coating is thinner than 1 μm, the solenoid-type induction heating device may not be able to rapidly heat the steel sheet S. If the thickness of the iron-based coating is thicker than 50 μm, the formation of a thick iron-based coating may reduce the steel sheet production efficiency of the entire facility. The iron-based coating has a composition containing, by mass%, 10% or less of at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder consisting of Fe and unavoidable impurities.

[0020] (Leveler) In an electrolytic iron-based plating apparatus 5, the shorter the distance between the steel sheet S and the electrode plates 8, the more improved the electrodeposition efficiency (plating efficiency). However, if the steel sheet S has a shape irregularity, such as warpage, in the thickness direction of the steel sheet S, the steel sheet S may come into contact with the electrode plates 8, causing scratches on the steel sheet S and resulting in quality problems. In addition, the distance between the steel sheet S and each electrode plate 8 may change, and the thickness of the iron-based plating formed on the surface of the steel sheet S may change depending on the change in distance. To avoid these problems, it is preferable to provide a leveler 10 on the entrance side of the iron-based plating apparatus 5 in the conveying direction of the steel sheet S, as shown in FIG. 1.

[0021] The leveler 10 is not limited, and examples of the leveler 10 include conventionally known roller-type and tension-type levelers. The leveler 10 shown in FIG. 1 is a roller-type leveler, in which upper rolls located above the steel sheet S and lower rolls located below the steel sheet S are alternately arranged in the conveying direction of the steel sheet S. The leveler 10 passes the steel sheet S between the rolls arranged in a staggered pattern, and repeatedly bends and unbends the steel sheet S. In this manner, the shape of the steel sheet S is corrected and flattened. In a tension-type leveler, roller levelers are arranged between tension rolls in the conveying direction, and the steel sheet S is passed through the roller leveler while tension is applied to the steel sheet S by the tension rolls, and repeatedly bends and unbends the steel sheet S. In this manner, the shape of the steel sheet S is corrected and flattened. Note that the presence or absence of a leveler may be selected as appropriate depending on the shape of the steel sheet S to be annealed in the continuous annealing equipment 1, the thickness of the iron-based coating to be formed on the surface of the steel sheet S, the specifications of the steel sheet to be manufactured in the continuous annealing equipment 1, etc.

[0022] (Plating thickness measuring device) In the example shown in FIG. 1, a transmission-type thickness gauge 11 functioning as a coating thickness measuring device is provided on the outlet side of the iron-based plating device 5 in the conveying direction of the steel sheet S. The transmission-type thickness gauge 11 irradiates the steel sheet S with radiation and measures the amount of radiation transmitted through the steel sheet S to determine the amount of radiation attenuation by the steel sheet S, and converts this amount of radiation attenuation into the thickness of the iron-based plating to measure the thickness of the iron-based plating. This is to adjust the voltage difference between the electrodes of the iron-based plating device 5, the current density, and the output of the rapid heating device (described later) based on the thickness of the iron-based plating formed on the surface of the steel sheet S. Methods for adjusting the output of the iron-based plating device 5 and the rapid heating device will be described later. Examples of radiation include X-rays and gamma rays.

[0023] The thickness of the iron-based coating may be measured by an X-ray fluorescence analyzer instead of the transmission-type thickness gauge 11. When measuring the thickness of the iron-based coating by an X-ray fluorescence analyzer, an X-ray fluorescence analyzer is disposed at the entrance and exit of the iron-based plating device 5 in the conveying direction of the steel sheet S. Each X-ray fluorescence analyzer irradiates the surface of the steel sheet S with X-rays, and the amount of elements other than iron on the surface of the steel sheet S at the entrance and exit of the iron-based plating device 5 is measured using the resulting X-ray fluorescence spectrum. The difference in the amount of elements other than iron between the entrance and exit of the iron-based plating device 5 is calculated, and the difference is converted into the thickness of the iron-based coating to measure the thickness of the iron-based coating. Examples of the amount of elements other than iron include carbon, silicon, and manganese.

[0024] In the continuous annealing equipment 1 shown in FIG. 1, on the outlet side of the looper 4, a heating zone 12, a soaking zone 14, a rapid heating zone 15, and a cooling zone 16 are arranged in this order in the conveying direction of the steel sheet S.

[0025] (heating zone) The heating zone 12 is a facility for heating the steel sheet S to a preset temperature within a temperature range of approximately 600°C to 900°C depending on the chemical composition of the steel sheet S. The heating zone 12 is provided with a heating device for heating the steel sheet S, and examples of the heating device include a direct flame or radiant combustion burner. The atmosphere around the steel sheet S in the heating zone 12 is not limited. The process of heating the steel sheet S in the heating zone 12 described above corresponds to the heating process of this embodiment.

[0026] (Pre-tropical zone) In the example shown in FIG. 1 , a preheating zone 13 is installed upstream of the heating zone 12 in the transport direction of the steel sheet S. The preheating zone 13 is equipment that preheats the steel sheet S as it enters the heating zone 12. In the preheating zone 13, for example, if the steel sheet S is a thin steel sheet, the steel sheet S is preheated to about 200°C. The method for heating the steel sheet S in the preheating zone 13 is not limited, but may be a heating method that utilizes high-temperature combustion exhaust gas generated in the heating zone 12.

[0027] (Soaking temperature) A soaking zone 14 is provided downstream of the heating zone 12 in the transport direction of the steel sheet S. The soaking zone 14 is a facility for maintaining the steel sheet S heated in the heating zone 12 at a predetermined temperature or for gently heating the steel sheet S. The heating device provided in the soaking zone 14 is not limited, but may be a heating device capable of supplementing the heat dissipated by the furnace body. Examples of such a heating device include a radiant tube type heating device. In this embodiment, a reducing gas supply source (not shown) is connected to the soaking zone 14, and reducing gas is supplied from the supply source to the soaking zone 14 to create a reducing atmosphere. This is to suppress oxidation of the iron-based coating formed on the surface of the steel sheet S and prevent a decrease in the adhesion of the zinc coating and the occurrence of pressing scratches due to the adhesion of iron oxides to the furnace rolls. Examples of the reducing gas include a mixture of nitrogen gas and hydrogen gas. The process of maintaining the steel sheet S at a predetermined temperature in the soaking zone 14 corresponds to the soaking process in this embodiment.

[0028] (rapid heating zone) A rapid heating zone 15 is provided downstream of the soaking zone 14 in the transport direction of the steel sheet S. The rapid heating zone 15 is equipment that rapidly heats the steel sheet S to a target annealing temperature (sometimes referred to as a target cooling start temperature). In order to control the mechanical properties of the steel sheet S, the target cooling start temperature of the steel sheet S in the cooling zone 16, which will be described later, is preferably in the range from the ferrite / austenite two-phase region to the austenite single-phase region, and specifically, it is preferably controllable in the range of 750°C to 900°C. As a rapid heating device to be installed in the rapid heating zone 15, an induction heating type heating device with high responsiveness to temperature control (sometimes referred to as thermal responsiveness) is preferred. In this embodiment, in order to suppress temperature variation in the width direction of the steel sheet S, a solenoid-type induction heating device (hereinafter simply referred to as an induction heating device) 15A is installed in the rapid heating zone 15 as the rapid heating device.

[0029] The induction heating device 15A may be a conventionally known device having a coiled heating wire (hereinafter referred to as a heating coil) and configured to pass the steel sheet S inside the heating coil. In the induction heating device 15A, when a high-frequency current is applied to the heating coil, an alternating magnetic flux is generated inside the heating coil along the conveyance direction of the steel sheet S. Then, an induced current is generated in the steel sheet S in a direction perpendicular to the alternating magnetic flux, and the steel sheet S is heated by the Joule heat generated at that time. Therefore, when the steel sheet S is rapidly heated by the induction heating device 15A, it is preferable that the steel sheet S is magnetic. Therefore, in this embodiment, an iron-based plating made of a magnetic ferrite phase is applied to the surface of the steel sheet S by the iron-based plating device 5.

[0030] The heating rate of the steel sheet S in the rapid heating zone 15 is preferably 10°C / s or more and 200°C / s or less. If the heating rate is less than 10°C / s, the α-phase crystal grains may become coarse, possibly resulting in a deterioration in the mechanical properties of the steel sheet S. If the heating rate is greater than 200°C / s, local high-temperature areas may occur in the width direction of the steel sheet S, making it impossible to heat the steel sheet S uniformly in the width direction. Furthermore, the heating rate of the steel sheet S in the rapid heating zone 15 is more preferably 20°C / s or more and 100°C / s or less. If the heating rate is 20°C / s or more, the line length of the continuous annealing equipment 1 can be shortened. If the heating rate is 100°C / s or less, the risk of buckling deformation of the steel sheet S due to thermal stress can be reduced.

[0031] When the steel sheet S is rapidly heated to the target annealing temperature in the rapid heating zone 15, the transformation from the ferrite phase to the austenite phase may not reach equilibrium immediately after heating of the steel sheet S, i.e., at or immediately after reaching the target annealing temperature. Therefore, if the steel sheet S is maintained at a temperature close to the target annealing temperature for too long, the transformation from the ferrite phase to the austenite phase may proceed more than necessary, complicating material quality control. Therefore, after reaching the target annealing temperature, it is preferable to promptly enter the steel sheet S into the cooling zone 16 and start cooling within 10 seconds, and more preferably within 5 seconds. Note that the rapid heating zone 15 is preferably installed immediately before the cooling zone 16, regardless of the structure of the soaking zone 14. In other words, when adding a rapid heating zone 15 to an existing continuous annealing facility, it is preferable to install the rapid heating zone 15 at the connection between the existing soaking zone and the cooling zone in the steel sheet transport direction. The process of rapidly heating the steel sheet S in the rapid heating zone 15 corresponds to the rapid heating process of this embodiment.

[0032] A cooling zone 16 is provided downstream of the rapid heating zone 15 in the conveying direction of the steel sheet S. The cooling zone 16 is equipment for cooling the steel sheet S to a predetermined temperature. The cooling zone 16 may be configured with only a first cooling zone 16A depending on the cooling amount and sheet passing speed of the steel sheet S required to obtain target mechanical properties, or may be configured with two stages including a first cooling zone 16A and a second cooling zone 16B as shown in FIG. 1. The cooling means for the steel sheet S in the cooling zone 16 is not limited, but examples of the cooling means include gas jet cooling, roll cooling, and water cooling. Note that, when the cooling zone 16 is configured with a first cooling zone 16A and a second cooling zone 16B as shown in FIG. 1, different cooling means may be used in the first cooling zone 16A and the second cooling zone 16B. Alternatively, the same type of cooling means may be used in the first cooling zone 16A and the second cooling zone 16B, but the cooling conditions may be different. The cooling rate of the steel sheet S in the cooling zone 16 is not limited, but is preferably 10 to 100°C / s. This is to prevent ferrite transformation during cooling due to an insufficient cooling rate, and to prevent rapid bainite transformation after cooling is stopped due to an excessive cooling rate. The step of cooling the steel sheet S in the cooling zone 16 described above corresponds to the cooling step of this embodiment.

[0033] (plated strip) The steel sheet S that has passed through the continuous annealing equipment 1 is transported to a subsequent process. In the example shown in FIG. 1 , a galvanizing zone 17 is provided downstream of the cooling zone 16 in the transport direction of the steel sheet S. An example of the coating process in the coating zone 17 is hot-dip galvanizing. In the hot-dip galvanizing process, the surface of the steel sheet S is coated with zinc by continuously immersing the steel sheet S in a zinc coating bath containing molten zinc. In this way, a galvanized steel sheet is produced. The above-mentioned coating zone 17 corresponds to the galvanizing zone of this embodiment.

[0034] In addition, alloying treatment equipment (not shown) may be provided downstream of the coating zone 17 in the conveying direction of the steel sheet S. The alloying treatment equipment is equipment that heats the steel sheet and performs an alloying treatment. Furthermore, downstream of the alloying equipment in the conveying direction of the steel sheet S, a holding zone, a final cooling zone, temper rolling equipment, a straightener, an outlet looper, a tension reel, and the like may be provided in order to improve the quality of the final product, the manufacturing stability, and efficiency. These pieces of equipment may be conventionally known equipment and are not limited thereto. These pieces of equipment are provided or used depending on the quality required of the final product.

[0035] (Metamorphosis rate meter) A transformation rate meter 18 is provided between the rapid heating zone 15 and the cooling zone 16 in the transport direction of the steel sheet S to measure the austenite phase fraction (sometimes referred to as the transformation rate) of the steel sheet S that has been heated in the rapid heating zone 15 and reached the target annealing temperature. This is for the purpose of adjusting the operating conditions of the rapid heating zone 15 based on the phase fraction of the steel sheet S to manufacture a steel sheet S with target mechanical properties. In other words, the phase fraction is used for feedback control of the operating conditions of the rapid heating zone 15. Therefore, in order to improve the accuracy of the feedback control, it is preferable to install the transformation rate meter 18 as close as possible to the rapid heating device between the rapid heating zone 15 and the cooling zone 16 in the transport direction of the steel sheet S.

[0036] The transformation rate meter 18 is not limited, but examples of the transformation rate meter 18 include a magnetic transformation rate measurement device and an X-ray diffraction device. The phase fraction of the steel sheet S may be measured using one of these devices. When a magnetic transformation rate measurement device is used as the transformation rate meter 18, the magnetic transformation rate measurement device may be the device described in JP 2019-7907 A. Furthermore, if the magnetic transformation rate measurement device is installed near the rapid heating zone 15 having the induction heating device 15A, the accuracy of measuring the phase fraction may be reduced due to the influence of the magnetic field generated by the induction heating device 15A of the rapid heating zone 15. Therefore, when a magnetic transformation rate measurement device is used as the transformation rate meter 18, it is preferable to install the magnetic transformation rate measurement device inside the soaking zone 14, at the entrance or exit of the cooling zone 16, or elsewhere that is less susceptible to the influence of the magnetic field generated by the induction heating device 15A of the rapid heating zone 15.

[0037] An X-ray diffractometer measures the phase fraction based on the diffraction peaks that appear when X-rays are irradiated onto the steel sheet S. When X-rays are irradiated onto the steel sheet S, diffraction peaks appear at the characteristic diffraction angles due to the crystalline structure of the austenite phase. Similarly, diffraction peaks appear at the characteristic diffraction angles due to the crystalline structure of the ferrite phase. The intensities and diffraction angles of these diffraction peaks are compared with data stored in a database to measure the phase fraction of the steel sheet S. Because the X-ray diffractometer measures the phase fraction of the steel sheet S based on this principle, it is less susceptible to the magnetic field generated by the rapid heating device than a magnetic transformation rate measurement device. Therefore, the X-ray diffractometer can be installed closer to the rapid heating device. Therefore, an X-ray diffractometer is preferable to a magnetic transformation rate measurement device in terms of improving the accuracy of feedback control of the operating conditions in the rapid heating zone 15. An example of an X-ray diffractometer is the X-CAP manufactured by SMS.

[0038] (Control device) The continuous annealing equipment 1 has a control device 19 that controls the various devices described above. FIG. 2 is a diagram for explaining the configuration of the control device 19 shown in FIG. 1. The control device 19 shown in FIG. 2 has a plating control unit 20, a rapid heating control unit 21, a phase fraction prediction model generation unit 22, and a storage unit 23. The control device 19 is mainly configured, for example, with a microcomputer. The control device 19 executes a program read from the storage unit 23, causing the control device 19 to function as the plating control unit 20, the rapid heating control unit 21, and the phase fraction prediction model generation unit 22.

[0039] The storage unit 23 may be, for example, an updatable flash memory, a built-in hard disk or a hard disk connected via a data communication terminal, an information recording medium such as a memory card, or a read / write device thereof. The storage unit 23 stores programs for the control device 19 to execute various functions, data used by the programs, and the like.

[0040] The storage unit 23 further stores a database 24 and a phase fraction prediction model 25. The database 24 stores various data to be input to the control device 19. Examples of the data to be input to the database 24 include operational performance data of the continuous annealing equipment 1, the phase fraction of the steel sheet S produced by the continuous annealing equipment 1, and the thickness of the iron-based coating.

[0041] Furthermore, a phase fraction prediction model 25 created in advance by the phase fraction prediction model generation unit 22 is stored in the storage unit 23. An example of the phase fraction prediction model 25 is a trained machine learning model trained by machine learning. The generation of the phase fraction prediction model 25 will be described later.

[0042] (Plating Control Department) The plating control unit 20 controls the iron-based plating device 5 based on the thickness of the iron-based coating measured by the transmission-type thickness gauge 11 to change the thickness of the iron-based coating formed on the surface of the steel sheet S. The thickness of the iron-based coating can be changed depending on the voltage difference between the anode and the cathode, the current density, and the conveying speed of the steel sheet S. When the continuous annealing equipment 1 starts operation, the above-mentioned parameters may be set to design-defined values ​​as default values. Thereafter, the parameters may be changed based on the thickness of the iron-based coating measured by the transmission-type thickness gauge 11. For example, if the thickness of the iron-based coating measured by the transmission-type thickness gauge 11 is thinner than a predetermined threshold, the voltage difference may be increased, the current density may be increased, or the conveying speed of the steel sheet S may be decreased. By doing so, the thickness of the iron-based coating is increased. The plating control unit 20 can change the thickness of the iron-based coating by changing at least one of the above-mentioned parameters.

[0043] (rapid heating control unit) The rapid heating control unit 21 controls the frequency of the high-frequency current applied to the induction heating device 15A in the rapid heating zone 15 and the output of the induction heating device 15A. When the continuous annealing equipment 1 starts operation, the frequency of the high-frequency current applied to the induction heating device 15A is determined based on actual data on the operating conditions of the continuous annealing equipment 1, or the high-frequency current frequency calculated from a predetermined table may be set as a default value. The output of the induction heating device 15A indicates the heating rate of the steel sheet S by the induction heating device 15A. The output of the induction heating device 15A is determined based on the size of the steel sheet S, the thickness of the iron-based coating, the line speed (LS) of the continuous annealing equipment 1, the temperature of the steel sheet S at the outlet side of the soaking zone 14 (i.e., the temperature of the steel sheet S at the inlet side of the induction heating device 15A), and the target annealing temperature of the steel sheet S. When the induction heating device 15A in the rapid heating zone 15 starts heating the steel sheet S, the rapid heating control unit 21 feedforward controls the frequency of the high-frequency current applied to the induction heating device 15A based on the thickness of the iron-based coating on the steel sheet S. The rapid heating control unit 21 also feedback-controls the output of the induction heating device 15A based on the phase fraction and the thickness of the iron-based coating on the steel sheet S. For example, if the thickness of the iron-based coating measured by the transmission-type thickness gauge 11 is thinner than a predetermined threshold, the output of the induction heating device 15A is increased to rapidly heat the steel sheet S. The induction heating device 15A also controls the output so that the difference between the actual measured temperature of the steel sheet S and the target temperature gradually decreases. The frequency of the high-frequency current applied to the induction heating device 15A in the rapid heating zone 15 is preferably 100 to 1000 kHz. This is because a frequency above 1000 kHz increases the cost of the power supply, and a frequency below 100 kHz may reduce the heating efficiency of the steel sheet S. The frequency and output of the induction heating device 15A are controlled independently of each other.

[0044] (Phase fraction prediction model generation unit) The phase fraction of the steel sheet S may be calculated using a phase fraction prediction model 25 generated by the phase fraction prediction model generation unit 22. The phase fraction prediction model 25 is a trained machine learning model that receives as input data (a) the alloy composition and dimensions of the steel sheet S, (b) the operating conditions of the continuous annealing equipment 1, and (c) the temperature history of the steel sheet S, and outputs as output data the phase fraction of the steel sheet S during annealing. The phase fraction prediction model 25 is generated by the phase fraction prediction model generation unit 22. The phase fraction prediction model generation unit 22 prepares a plurality of training data (training data) from various performance data stored in the database 24. The training data is a data set that receives as input data the performance data of the operating conditions of the continuous annealing equipment 1, the alloy composition and dimensions of the steel sheet S, and the temperature history during the annealing process of the steel sheet S, and outputs as output data the phase fraction corresponding to the operating conditions and temperature history. The phase fraction prediction model 25 is then generated by machine learning using the plurality of training data.

[0045] (a) Alloy composition and dimensions of steel plate S The alloy composition of the steel sheet S refers to the components contained in the steel sheet S and is information that determines the Ar1 transformation point and the Ar3 transformation point. The dimensions of the steel sheet S refer to the thickness of the steel sheet S that is annealed in the continuous annealing equipment 1, and this thickness is information that affects the heating and cooling rates of the steel sheet S during annealing.

[0046] (b) Operating conditions of continuous annealing equipment 1 The operating conditions refer to various setting values ​​that are set in the continuous annealing equipment 1 when the steel sheet S is annealed by the continuous annealing equipment 1. Of the operating conditions used as input data for the phase fraction prediction model 25, an essential item is the conveying speed of the steel sheet S conveyed inside the continuous annealing equipment 1. This is because the conveying speed of the steel sheet S affects annealing conditions such as the heating rate, cooling rate, and soaking time. For this reason, the conveying speed of the steel sheet S is included as input data for the phase fraction prediction model 25.

[0047] In addition to the conveying speed of the steel sheet S, the operational conditions used as input data for the phase fraction prediction model 25 can include, for example, the rolling reduction setting during the hot rolling process, the cooling start temperature at the hot rolling run-out table, and the cooling end temperature at the hot rolling run-out table. Other operational conditions include the coiling temperature of the hot rolled coil, the pass schedule in the cold rolling process, the set furnace temperature in the continuous annealing equipment 1, and the amount of gas input to the burner. At least one of these items may be included as an operational condition. By including at least one of these items, it becomes possible to grasp the processing rate of the steel sheet S, i.e., the amount of processing strain before annealing, and the microstructural state of the steel sheet S before annealing. This improves the prediction accuracy of the transformation temperature and phase fraction. Among the operational conditions described above used as input data for the phase fraction prediction model 25, it is particularly effective to include the coiling temperature of the hot rolled coil. This is because the coiling temperature of the hot rolled coil affects the grain size of the ferrite phase at the inlet side of the annealing furnace, changing the transformation rate to the austenite phase. Furthermore, the grain size of the ferrite phase varies not only with the coiling temperature but also with the cold rolling pass schedule. Therefore, it is particularly effective to include the cold rolling pass schedule as input data for the phase fraction prediction model 21. Furthermore, the operating conditions may include the cold rolling reduction, which is a useful variable. The pass schedule refers to the set value of the reduction amount during rolling, that is, the target reduction amount.

[0048] (c) Temperature history of steel plate S The temperature history of the steel sheet S is the surface temperature of the steel sheet S measured inside the continuous annealing equipment 1. Of the temperature history of the steel sheet used as input data for the phase fraction prediction model 25, essential items are the temperature of the steel sheet S immediately before the start of heating and the maximum temperature reached by the rapid heating device. The temperature of the steel sheet S immediately before the start of heating may be the temperature of the steel sheet S measured at the entry side of the continuous annealing equipment 1, the entry side of the heating zone, the entry side of the soaking zone, or the entry side of the rapid heating device. Note that the more points at which the temperature of the steel sheet S is measured, the more accurately the transformation temperature can be determined, thereby improving the accuracy of prediction of the phase fraction by the phase fraction prediction model 25. Although not shown, a thermometer is installed at each of the above-mentioned temperature measurement points.

[0049] Here, the temperature history of the steel sheet S may be temperature data measured at the connection points between the devices constituting the continuous annealing equipment 1, or discrete temperature data obtained by calculation. Furthermore, to improve the prediction accuracy of the phase fraction of the steel sheet S by the phase fraction prediction model 25 and to improve the production yield in the continuous annealing equipment 1, for example, a model that predicts the surface temperature of the steel sheet S at locations where it has been difficult to measure the temperature of the steel sheet S in the past may be introduced. In the following description, this model will be referred to as a steel sheet temperature prediction model. The steel sheet temperature prediction model may be a physical model for numerical analysis or a machine learning model. When the steel sheet temperature prediction model is a machine learning model, training data of a data set described below may be used to generate the steel sheet temperature prediction model. The training data is a data set in which the operating conditions of the continuous annealing equipment 1, the alloy composition of the steel sheet, and the cross-sectional shape of the steel sheet S are input data, and the temperature of the steel sheet S at the above-mentioned locations inside the continuous annealing equipment 1 where it is difficult to measure the temperature is output data. Examples of the operating conditions of the continuous annealing equipment 1 include the line speed, i.e., the conveying speed of the steel sheet S, and furnace temperatures at multiple locations in the heating zone 12 and the soaking zone 14. Examples of the cross-sectional shape of the steel sheet S include the thickness and width of the steel sheet S. A steel sheet temperature prediction model is generated by machine learning using such training data. Furthermore, the input data for the steel sheet temperature prediction model may include information on upstream processes of the continuous annealing equipment 1. Examples of the information on the upstream processes include rolling conditions in the hot rolling process, including the reheating temperature and pass schedule of the steel sheet S, and rolling conditions in the cold rolling process, including the pass schedule. Examples of the information on the upstream processes include the cooling start temperature and cooling stop temperature at the finish exit side in the hot rolling process, and cooling conditions such as the coil winding temperature. The information on the upstream processes may include at least one of these pieces of information.

[0050] In this embodiment, the phase fraction of the steel sheet S that has been actually measured is used as the output data of the training data for the phase fraction prediction model 25. The method for measuring the phase fraction is not limited, but the phase fraction may be determined by conducting an annealing experiment on a steel sheet under annealing conditions that are substantially the same as the operating conditions of the existing continuous annealing equipment 1, and then observing the structure of the cross section of the steel sheet that underwent the annealing experiment. Furthermore, by conducting multiple annealing experiments with different annealing conditions, data on the phase fraction under various annealing conditions may be created. The various annealing conditions may then be used as input data for the training data, and the phase fraction data under each annealing condition may be used as output data for the training data.

[0051] Since the phase fraction predicted by the phase fraction prediction model 25 is the phase fraction at the maximum temperature reached in the rapid heating zone 15, it is preferable to rapidly cool the steel sheet that has reached the maximum temperature in the annealing experiment described above to freeze the structure and determine the phase fraction by observing the cross-sectional structure. Instead of determining the phase fraction by experiment, the steel sheet S may be annealed by changing the operating conditions in an existing continuous annealing facility 1, and the cooled steel sheet may be sampled and the phase fraction determined by observing the cross-section of the steel sheet. Alternatively, the phase fraction may be determined by numerical analysis using a physical model. Either method can determine the thermal history and phase fraction of the steel sheet S during annealing under the operating conditions of the continuous annealing facility 1. Note that the phase fraction of the steel sheet during annealing described above refers to the phase fraction of the steel sheet from immediately after the end of annealing in the rapid heating device to immediately before the start of cooling in the cooling zone.

[0052] (Actions and Effects) In this embodiment, an iron-based plating composed mainly of a ferrite phase is applied to the surface of the steel sheet S by the iron-based plating device 5, and the steel sheet S then enters the induction heating device 15A. When a high-frequency current is applied to the heating coil of the induction heating device 15A, an alternating magnetic flux is generated along the conveyance direction of the steel sheet S. An induced current is then generated along the width direction, i.e., on the surface of the steel sheet S, in a direction perpendicular to the alternating magnetic flux. The Joule heat generated thereby heats the surface of the steel sheet S. Furthermore, heat conduction from the surface of the steel sheet S to the interior of the steel sheet S allows the entire steel sheet S to be rapidly and uniformly heated to the target temperature. In this manner, the steel sheet S can be rapidly heated to the target cooling start temperature in the width direction and substantially uniformly over the entire length of the steel sheet S. Therefore, by subsequently cooling the steel sheet S in the cooling zone 16, a steel sheet S with the desired mechanical properties can be manufactured. The method of annealing the steel sheet S in the continuous annealing equipment 1 of this embodiment described above corresponds to the continuous annealing method of this embodiment. The process of annealing the steel sheet S in the continuous annealing equipment 1 of this embodiment and galvanizing it with the galvanizing strip 17 to produce a galvanized steel sheet corresponds to the method of producing a galvanized steel sheet of this embodiment. [Example]

[0053] An experiment was conducted to anneal DP steel (dual phase steel, hereinafter simply referred to as steel sheet) using a continuous annealing line configured similarly to the continuous annealing line 1 shown in Figure 1, to see whether the target mechanical properties could be obtained. The steel sheet described above was in a coil form, and was unwound by a payoff reel and supplied to the continuous annealing line.

[0054] The above-mentioned steel sheet is not limited as long as it is necessary to control the two-phase structure of ferrite and austenite to a single-phase austenite region, but preferably has the following components: C 0.050 to 0.250% or less, Si 0.010 to 2.000% or less, Mn 0.500 to 4.000% or less, P 0.100% or less, S 0.050% or less, sol. Al 0.005 to 0.100% or less, and N 0.100% or less. Furthermore, the steel sheet may optionally contain Cr, Cu, Ni, Sb, Sn (each 1.000% or less), Mo, V, Ti, Nb (each 0.500% or less), Ta (0.100% or less), Mg, Zr (each 0.050% or less), B, Ca, and REM (each 0.005% or less). The balance of the steel sheet has a chemical composition consisting of Fe and inevitable impurities.

[0055] Table 1 summarizes the manufacturing conditions and evaluation results for Examples 1 to 3 and Comparative Examples 1 to 4. In Examples 1 to 3 and Comparative Examples 1 to 4, 30 coils of each product were manufactured at multiple strength levels (sometimes referred to as grades) to examine the variation in the mechanical properties of the products. Three strength levels (980 MPa, 1180 MPa, and 1470 MPa) were manufactured, and 10 products of each strength level (1.0 mm, 1.5 mm, and 2.0 mm) were manufactured. These 10 products of each grade and thickness were all manufactured from slabs cast in different lots using a continuous casting machine. Therefore, although within the manufacturing control range, the chemical compositions of each product varied and the transformation behavior was not uniform. Table 1 also summarizes the mechanical properties of each product by grade. Therefore, the mechanical properties of each grade include the mechanical properties of products with different thicknesses.

[0056] [Table 1]

[0057] Each product was hot-rolled and pickled using a conventional method, and annealed and cold-rolled as necessary. Subsequently, heat treatment was performed using a conventionally known zinc-coated steel sheet manufacturing facility or a manufacturing facility configured substantially similarly to the manufacturing facility of this embodiment, followed by cooling and zinc plating. The sheet threading speed was 40 to 120 mpm. JIS No. 5 tensile test specimens were taken from multiple locations on the final zinc-coated steel sheet, and a tensile test was performed using these test specimens in accordance with JIS Z 2241:2023. Specifically, one coil was randomly selected from ten coils that were the final products, and the zinc-coated steel sheet was unwound from this coil. Test specimens were taken from the center of the zinc-coated steel sheet in the width direction and at regular intervals along the length, and a tensile test was performed using these test specimens. Test specimens were also taken at regular intervals along the width direction, and a tensile test was performed using these test specimens. The center in the width direction at the tip of the galvanized steel sheet unwound from the coil described above was set as a representative point of the galvanized steel sheet, and the deviation from the representative point was used to evaluate whether a galvanized steel sheet with the target mechanical properties had been manufactured.

[0058] Furthermore, for the 980 MPa, 1180 MPa, and 1470 MPa grades, the target mechanical properties, i.e., tensile strength (TS) ranges are 980 MPa or more, 1180 MPa or more, and 1470 MPa or more, respectively. Therefore, when the tensile strength measured by the tensile test is equal to or greater than the lower limit mentioned above for each grade, the target mechanical properties are achieved and the product is judged to be acceptable.

[0059] Example 1 Example 1 is an example in which a galvanized steel sheet was produced using a manufacturing facility configured substantially similarly to the manufacturing facility of this embodiment shown in FIG. 1. That is, a solenoid-type induction heating device was provided at the outlet of the soaking zone, and an iron-based plating device was provided upstream of the solenoid-type induction heating device in the steel sheet transport direction. In addition, the phase fraction and the thickness of the iron-based plating were predicted from the steel sheet composition and the operating conditions of the manufacturing facility, and the solenoid-type induction heating device was manually controlled. As a result, the mechanical properties of all the samples were within the acceptable range.

[0060] Example 2 Example 2 is an example in which a transformation rate meter was provided as a phase fraction information acquisition device, and the solenoid-type induction heating device was automatically controlled based on the phase fraction measured by the transformation rate meter. Other than that, the example was the same as Example 1. In Example 2, the mechanical properties were all within the acceptable range, and the variation in mechanical properties between the coils was reduced compared to Example 1.

[0061] Example 3 Example 3 is an example in which the thickness of the iron-based coating was measured, and the solenoid-type induction heating device was automatically controlled based on the measured thickness of the iron-based coating. Other than that, the example was the same as Example 2. In Example 3, the mechanical properties were all within the acceptable range, and the variation in mechanical properties between coils and in the width direction within the same coil was reduced compared to Examples 1 and 2.

[0062] (Comparative Example 1) Comparative Example 1 is an example in which a galvanized steel sheet was produced using conventional continuous annealing equipment. That is, the continuous annealing equipment was not provided with a solenoid-type induction heating device that functions as a rapid heating zone. Furthermore, the heating method for the steel sheet in the continuous annealing equipment was a conventional radiant tube method. Therefore, in Comparative Example 1, the thermal response during sheet threading was poor compared to Examples 1 to 3. As a result, the variation in the longitudinal tensile strength of the steel sheet within the same coil was greater than in Examples 1 to 3, and some of the steel sheets were rejected.

[0063] (Comparative Example 2) Comparative Example 2 is an example in which an iron-based plating device was installed upstream of the soaking zone in the conveying direction of the steel sheet in the continuous annealing equipment of Comparative Example 1, and the steel sheet was iron-based plated. However, in the case of heating using the radiant tube method, the effect of the iron-based plating was not obtained. In other words, the steel sheet could not be rapidly heated. As a result, similar to Comparative Example 1, the steel sheet was partially rejected.

[0064] (Comparative Example 3) Comparative Example 3 is an example in which a solenoid induction heating device was installed as a rapid heating zone between the soaking zone and the cooling zone in the continuous annealing equipment of Comparative Example 1, and a transformation rate meter was installed on the outlet side of the solenoid induction heating device. The solenoid induction heating device was controlled based on the phase fraction measured by the transformation rate meter. The solenoid induction heating device significantly reduced the heating efficiency for the 1180 MPa class and 1470 MPa class steel sheets, which have high austenite fractions. As a result, the 1180 MPa class and 1470 MPa class steel sheets could not be heated to the target cooling start temperature, resulting in failure.

[0065] Comparative Example 4 Comparative Example 4 is an example in which a transverse induction heating device was installed as a rapid heating zone between the soaking zone and the cooling zone in the continuous annealing equipment of Comparative Example 1. A transverse induction heating device can rapidly heat steel sheets with a high austenite phase fraction more efficiently than a solenoid induction heating device. Therefore, the variation in tensile strength in the longitudinal direction of the steel sheet was within the acceptable range. However, the variation in tensile strength in the width direction within the same coil became large, resulting in a failure. [Explanation of symbols]

[0066] 1. Continuous annealing equipment 2 Payoff Reel 3 Welder 4 Looper 5. Iron-based plating equipment 6 Conductor Roll 7 Backup Role 8 Electrode plate 9 Passage 10 Leveller 11 Transmission type plate thickness meter (plating thickness measuring device) 12 Heating Zone 13 Pre-tropical zone 14. Equal Temperature 15 Rapid heating zone 15A solenoid type induction heater 16 Cooling Zone 16A First cooling zone 16B Second cooling zone 17 Galvanized strip (galvanized strip) 18 Metamorphosis Rate Meter 19 Control device 20 Plating Control Section 21 Rapid heating control section 22 Phase fraction prediction model generation unit 23 Memory section 24 databases 25 Phase fraction prediction model S steel plate

Claims

1. A continuous annealing facility for steel sheets, comprising a heating zone, a soaking zone, a rapid heating zone, and a cooling zone arranged in this order in a conveying direction of the steel sheet, a plating device that applies an iron-based plating to the steel sheet upstream of the heating zone in the conveying direction; a solenoid type induction heating device provided in the rapid heating zone.

2. a plating thickness measuring device for measuring the thickness of the iron-based plating on the steel sheet; 2. The continuous annealing equipment for steel sheets according to claim 1, further comprising a control device that controls at least one of the plating device and the solenoid type induction heating device based on a thickness of the iron-based coating.

3. The method further comprises a transformation rate meter for measuring a phase fraction of the steel sheet, the control device further controls the rapid heating zone based on the thickness of the iron-based coating and the phase fraction; 3. The continuous annealing equipment for steel sheets according to claim 2, wherein the transformation rate meter is provided between the rapid heating zone and the cooling zone in the conveying direction.

4. 4. The continuous annealing equipment for steel sheets according to claim 1, wherein the iron-based coating has a component composition containing, by mass%, 10% or less of at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder consisting of Fe and unavoidable impurities.

5. The continuous annealing equipment for steel sheets according to any one of claims 1 to 3, further comprising a galvanizing zone for galvanizing the steel sheets downstream of the cooling zone in the conveying direction.

6. A continuous annealing method for a steel sheet, comprising subjecting a steel sheet to a heating step, a soaking step, a rapid heating step, and a cooling step in this order, a plating step of applying an iron-based plating to the steel sheet by a plating device; In the rapid heating step, the steel sheet having the iron-based plating applied thereto is heated by a solenoid-type induction heating device.

7. measuring the thickness of the iron-based plating of the steel sheet; 7. The method for continuous annealing a steel sheet according to claim 6, wherein at least one of the plating device and the solenoid type induction heating device is controlled based on the thickness of the iron-based coating.

8. measuring a phase fraction of the steel plate heated in the rapid heating step; The method for continuous annealing a steel sheet according to claim 7, wherein the solenoid type induction heating device is controlled based on the thickness of the iron-based coating and the phase fraction.

9. A method for producing a galvanized steel sheet, comprising: subjecting a steel sheet to continuous annealing by the method for continuous annealing a steel sheet according to any one of claims 6 to 8; and then galvanizing the steel sheet to produce a galvanized steel sheet.

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