Continuous annealing facility for steel sheet, continuous annealing method for steel sheet, and method for producing galvanized steel sheet

Abstract

To provide a continuous annealing facility for a steel sheet, a continuous annealing method for a steel sheet, and a method for producing a galvanized steel sheet, the facility and methods enabling rapid heating of the steel sheet substantially uniformly across the width direction to manufacture a steel sheet having desired mechanical characteristics.SOLUTION: There is provided a continuous annealing facility 1 for a steel sheet S in which a heating zone 5, a soaking zone 7, a rapid heating zone 9, and a cooling zone 10 are arranged in this order in a conveying direction of the steel sheet S, the facility comprising a humidifying device 8 that humidifies the interior of the soaking zone 7, a solenoid-type induction heating device 9A provided in the rapid heating zone 9, and a control device 14 that controls the humidifying device 8 to set a dew point inside the soaking zone 7 within a range of 5°C to 30°C.SELECTED DRAWING: Figure 1

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 a radiant tube heating device is used as the heating device for an annealing furnace, 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 be heated to 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, which has a heating zone, a soaking zone, a rapid heating zone, and a cooling zone arranged in this order in the conveying direction of the steel sheet, the facility comprising: a humidifier that humidifies the inside of the soaking zone; a solenoid-type induction heating device provided in the rapid heating zone; and a control device that controls the humidifier to set the dew point inside the soaking zone within a range of 5 to 30°C. [2] The continuous annealing equipment for steel sheets according to [1], 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 phase fraction, and the transformation rate meter is provided between the rapid heating zone and the cooling zone in the conveying direction. [3] A continuous annealing facility for steel sheets according to [1] or [2], further comprising a decarburization layer thickness information acquisition device that acquires decarburization layer thickness information of the steel sheet located between the soaking zone and the rapid heating zone in the conveying direction, wherein the control device controls the humidifier based on the decarburization layer thickness information. [4] The continuous annealing facility for steel sheets according to [2], further comprising a decarburization layer thickness information acquisition device that acquires decarburization layer thickness information of the steel sheet located between the soaking zone and the rapid heating zone in the conveying direction, wherein the control device controls the rapid heating zone and the humidifier based on the decarburization layer thickness information. [5] A continuous annealing facility for steel sheets according to any one of [1] to [4], further comprising a coating zone for coating the steel sheets downstream of the cooling zone in the conveying direction. [6] A continuous annealing method for steel sheet, which comprises subjecting a steel sheet to a heating step, a soaking step, a rapid heating step, and a cooling step in this order, wherein in the soaking step, the dew point around the steel sheet is controlled to within a range of 5 to 30°C, and in the rapid heating step, the steel sheet is heated by a solenoid-type induction heating device. [7] A method for continuous annealing of a steel sheet according to [6], wherein the 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 phase fraction. [8] A continuous annealing method for a steel sheet according to [6] or [7], wherein information on a decarburized layer thickness of the steel sheet after the soaking step and before being heated in the rapid heating step is acquired, and the dew point in the soaking step is controlled based on the information on the decarburized layer thickness. [9] The continuous annealing method for a steel sheet according to [7], further comprising: acquiring information on a decarburized layer thickness of the steel sheet after the soaking step and before the steel sheet is heated in the rapid heating step; and controlling the solenoid-type induction heating device in the rapid heating step and the dew point in the soaking step based on the information on the decarburized layer thickness and the phase fraction.

[10] 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 [6] to [9]; and then galvanizing the steel sheet to produce a galvanized steel sheet. [Effects of the Invention]

[0011] According to the present invention, when a steel plate is rapidly heated, the steel plate can be heated almost uniformly across its width, and a steel plate having the desired mechanical properties can be manufactured. [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 equipment 1 for steel sheets according to an embodiment of the present invention (hereinafter referred to as the present embodiment), the dew point in the soaking zone is controlled to generate a ferrite phase on the surface of the steel sheet. This allows the solenoid-type induction heating device in the rapid heating zone to efficiently rapidly heat the steel sheet, and also allows the temperature of the steel sheet to be brought to the target cooling start temperature almost uniformly in the width direction of the steel sheet. Therefore, when the steel sheet is subsequently cooled in the cooling zone, a hard structure can be generated in the steel sheet, and a steel sheet with the target mechanical properties can be produced.

[0014] Fig. 1 is a diagram illustrating the configuration of a continuous annealing line 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 along the conveying direction of the steel sheet S. In the example shown in Fig. 1, two payoff reels 2 are provided. The steel sheets S unwound from these payoff reels 2 are conveyed alternately to a welder 3, where the tail end of the preceding steel sheet S and the front end of the following steel sheet S are joined together by welding.

[0015] The looper 4 is a facility for ensuring an excess length of the steel sheet S so that the steel sheet S can be continuously passed through when the steel sheets S are joined together by the welder 3. After passing through the looper 4, the steel sheet S enters the continuous annealing facility 1. In the continuous annealing facility 1, a heating zone 5, a soaking zone 7, a rapid heating zone 9, and a cooling zone 10 are arranged in this order in the conveying direction of the steel sheet S.

[0016] (heating zone) A heating zone 5 is provided on the inlet side of the continuous annealing equipment 1. The heating zone 5 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 5 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. In addition, it is preferable that the atmosphere around the steel sheet S in the heating zone 5 is an oxidizing atmosphere. When the steel sheet S is heated in an oxidizing atmosphere, an iron oxide layer is formed on the outermost surface of the steel sheet S. Si contained in the steel sheet S is less likely to diffuse in the iron oxide layer than in the steel. Therefore, by heating the steel sheet S in an oxidizing atmosphere, it is possible to suppress the concentration of Si in the surface layer of the steel sheet S. This also facilitates the formation of a ferrite phase in the surface layer of the steel sheet S. The process of heating the steel sheet S in the heating zone 5 described above corresponds to the heating process of this embodiment.

[0017] (Pre-tropical zone) In the example shown in FIG. 1 , a preheating zone 6 is installed upstream of the heating zone 5 in the transport direction of the steel sheet S. The preheating zone 6 is equipment that preheats the steel sheet S as it enters the heating zone 5. In the preheating zone 6, 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 6 is not limited, but may be a heating method that utilizes high-temperature combustion exhaust gas generated in the heating zone 5.

[0018] (Soaking temperature) A soaking zone 7 is provided downstream of the heating zone 5 in the transport direction of the steel sheet S. The soaking zone 7 is equipment that maintains the steel sheet S heated in the heating zone 5 at a predetermined temperature or that gently heats the steel sheet S. The soaking zone 7 of this embodiment promotes decarburization near the surface of the steel sheet by controlling the dew point inside the soaking zone 7, i.e., the dew point around the steel sheet S. Furthermore, even if a solenoid-type induction heating device, which will be described later, is used, a ferrite phase with high heating efficiency is formed in the outermost layer of the steel sheet S. Here, controlling the dew point means controlling the moisture content inside the soaking zone 7. For this reason, the continuous annealing equipment 1 has a humidifying device that supplies humidified gas to the inside of the soaking zone 7 to humidify it.

[0019] The dew point in the soaking zone 7 is preferably in the range of 5 to 30°C. For example, if the dew point is below 5°C, the amount of moisture required for decarburization may be insufficient. Therefore, the dew point is preferably 5°C or higher. On the other hand, if the amount of moisture is excessive, the atmosphere in the continuous annealing equipment 1 becomes oxidizing. This makes it difficult to reduce iron oxides in the steel sheet S, which may result in a decrease in the galvanizability of the steel sheet S in the galvanizing strip described below. Furthermore, the rolls and furnace walls (not shown) inside the continuous annealing equipment 1 may also be damaged. Therefore, the dew point is preferably 30°C or lower. Furthermore, in the soaking zone 7, it is preferable to maintain the steel sheet S at a temperature between the Ar1 transformation point and the Ar3 transformation point. This is because maintaining the steel sheet S in the ferrite / austenite two-phase region improves the decarburization rate. The heating device provided in the soaking zone 7 is not limited, but may be a heating device that compensates for the heat dissipated from the furnace body. The process of maintaining the steel sheet S at a predetermined temperature in the soaking zone 7 corresponds to the soaking step in this embodiment.

[0020] (humidifier) The humidifier 8 may be any conventional humidifier 8 that can control the dew point of the soaking zone 7. Examples of the humidifier 8 include a hollow fiber membrane humidifier, a bubble humidifier, and a mist spray humidifier. The humidifier 8 may be any of the three types described above, but a hollow fiber membrane humidifier is less likely to introduce water droplets into the gas flow path that supplies humidified gas to the soaking zone 7. This allows for a wider range of control over the flow rate of the humidified gas supplied to the soaking zone 7, making it more preferable than the other two humidifiers. Regardless of which humidifier is used as the humidifier 8, it is preferable to maintain the temperature of the pipe that supplies humidified gas to the soaking zone 7 above the dew point to prevent condensation in the soaking zone 7.

[0021] The configuration of each humidifier will be explained. The hollow fiber membrane type humidifier is equipped with a hollow fiber membrane made of ion exchange resin. This device is designed so that the gas to be humidified flows inside the hollow fiber membrane and water flows outside the hollow fiber membrane. This allows water molecules to pass through the hollow fiber membrane and humidify the gas to be humidified. The gas to be humidified in this way (hereinafter sometimes referred to as humidified gas) is supplied to the equalizing zone 7 via the gas flow path.

[0022] A bubble-type humidifier is equipped with a tank that stores water. A gas pipe is placed in the water stored in the tank, and the gas to be humidified is released into the water from the gas pipe. In other words, the gas is humidified by passing it through water. The humidified gas is then supplied to the soaking zone 7 via the gas flow path.

[0023] The mist spray humidifier humidifies the gas by spraying mist of water onto the gas to be humidified. The humidified gas is supplied to the soaking zone 7 through a gas flow path.

[0024] (rapid heating zone) A rapid heating zone 9 is provided downstream of the soaking zone 7 in the transport direction of the steel sheet S. The rapid heating zone 9 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 10 described below 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 9, 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) 9A is installed in the rapid heating zone 9 as the rapid heating device.

[0025] The induction heating device 9A 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 9A, when a high-frequency current is applied to the heating coil, an alternating magnetic flux is generated inside the heating coil along the transport 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 9A, it is preferable that the steel sheet S is magnetic. For this reason, in this embodiment, the dew point in the soaking zone 7 is controlled to form a magnetic ferrite phase on the surface of the steel sheet S.

[0026] The heating rate of the steel sheet S in the rapid heating zone 9 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 9 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.

[0027] When the steel sheet S is rapidly heated to the target annealing temperature in the rapid heating zone 9, 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 10 and start cooling within 10 seconds, and more preferably within 5 seconds. Note that the rapid heating zone 9 is preferably installed immediately before the cooling zone 10, regardless of the structure of the soaking zone 7. In other words, when the rapid heating zone 9 is added to an existing continuous annealing facility, it is preferable to install the rapid heating zone 9 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 9 corresponds to the rapid heating process of this embodiment.

[0028] A cooling zone 10 is provided downstream of the rapid heating zone 9 in the conveying direction of the steel sheet S. The cooling zone 10 is equipment for cooling the steel sheet S to a predetermined temperature. The cooling zone 10 may be configured with only a first cooling zone 10A 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 10A and a second cooling zone 10B as shown in FIG. 1. The cooling means for the steel sheet S in the cooling zone 10 is not limited, but examples of the cooling means include gas jet cooling, roll cooling, and water cooling. Note that, when the cooling zone 10 is configured with a first cooling zone 10A and a second cooling zone 10B as shown in FIG. 1, different cooling means may be used in the first cooling zone 10A and the second cooling zone 10B. Alternatively, the same type of cooling means may be used in the first cooling zone 10A and the second cooling zone 10B, but the cooling conditions may be different. The cooling rate of the steel sheet S in the cooling zone 10 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 10 described above corresponds to the cooling step of this embodiment.

[0029] (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 coating zone 11 is provided downstream of the cooling zone 10 in the transport direction of the steel sheet S. An example of the coating treatment in the coating zone 11 is a hot-dip galvanizing treatment in which the steel sheet S is immersed in a galvanizing bath containing molten zinc. The steel sheet S that has passed through the continuous annealing equipment 1 is immersed in the galvanizing bath. In this way, the surface of the steel sheet S is plated with zinc, and a galvanized steel sheet is produced.

[0030] In addition, alloying treatment equipment (not shown) may be provided downstream of the coating zone 11 in the conveying direction of the steel sheet S. The alloying treatment equipment is equipment that heats the steel sheet and performs 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 stability of production, 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.

[0031] (Decarburized layer thickness information acquisition device) X-ray fluorescence analyzers 12 functioning as decarbonization amount information acquisition devices are provided on both sides of the soaking zone 7 in the transport direction of the steel sheet S. Each X-ray fluorescence analyzer 12 irradiates the surface of the steel sheet S with X-rays and measures the carbon amount on the surface of the steel sheet S from the fluorescent X-ray spectrum generated at that time. In this embodiment, the decarbonization amount measured by the X-ray fluorescence analyzer 12 is input to a control device described later. Then, the control device estimates the thickness of the decarbonization layer on the surface of the steel sheet S in the soaking zone 7 based on the difference between the carbon amount on the inlet side and the carbon amount on the outlet side of the soaking zone 7, the dew point of the soaking zone 7, the components of the steel sheet S, and the thickness of the iron oxide layer on the steel sheet S.

[0032] An X-ray diffractometer may be used as the decarbonization amount information acquisition device instead of the X-ray fluorescence analyzer 12. In this case, the X-ray diffractometer is provided on the outlet side of the soaking zone 7. The X-ray diffractometer measures the fraction of the ferrite phase or austenite phase (hereinafter referred to as the phase fraction) on the surface of the steel sheet S. The phase fraction is input to a control device, which estimates or calculates the decarburization layer thickness on the surface of the steel sheet S in the soaking zone 7 from the phase fraction, dew point, the composition of the steel sheet S, and the thickness of the iron oxide layer. In other words, the decarburization layer thickness information acquisition device may be either the X-ray fluorescence analyzer 12 or the X-ray diffractometer. It is sufficient that the decarburization layer thickness information acquisition device acquires decarburization layer thickness information of the steel sheet S located between the soaking zone 7 and the rapid heating zone 9 in the conveying direction of the steel sheet S, or of the steel sheet S after passing through the soaking zone 7 and before being rapidly heated in the rapid heating zone 9. The thickness of the iron oxide layer can be estimated from the detected intensity of the oxide on the surface of steel sheet S detected by an X-ray diffraction device. In addition, a model equation for the amount of oxidation can be created from the relationship between the atmospheric composition and temperature and the thickness of the iron oxide layer in offline laboratory tests, and the thickness of the oxide layer on the surface of steel sheet S can be estimated using this model equation.

[0033] (Metamorphosis rate meter) A transformation rate meter 13 is provided between the rapid heating zone 9 and the cooling zone 10 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 9 and reached the target annealing temperature. This is to adjust the operating conditions of the rapid heating zone 9 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 9. Therefore, in order to improve the accuracy of the feedback control, it is preferable to install the transformation rate meter 13 as close as possible to the rapid heating device between the rapid heating zone 9 and the cooling zone 10 in the transport direction of the steel sheet S.

[0034] The transformation rate meter 13 is not limited, but examples thereof 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 13, 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 9 having the induction heating device 9A, the accuracy of measuring the phase fraction may be reduced due to the influence of the magnetic field generated by the induction heating device 9A in the rapid heating zone 9. Therefore, when a magnetic transformation rate measurement device is used as the transformation rate meter 13, it is preferable to install the magnetic transformation rate measurement device inside the soaking zone 7, on the entrance or exit side of the cooling zone 10, or elsewhere that is less susceptible to the influence of the magnetic field generated by the induction heating device 9A in the rapid heating zone 9.

[0035] 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. Specifically, 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 9. An example of an X-ray diffractometer is the X-CAP manufactured by SMS.

[0036] (Control device) The continuous annealing equipment 1 has a control device 14 that controls the various devices described above. FIG. 2 is a diagram illustrating the configuration of the control device 14 shown in FIG. 1. The control device 14 shown in FIG. 2 has a humidification control unit 15, a rapid heating control unit 16, a decarburized layer thickness calculation unit 17, and a storage unit 19. The control device 14 is mainly configured, for example, with a microcomputer. The control device 14 executes a program read from the storage unit 19, causing the control device 14 to function as the humidification control unit 15, the rapid heating control unit 16, the decarburized layer thickness calculation unit 17, and the phase fraction prediction model generation unit 18.

[0037] The storage unit 19 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 19 stores programs for the control device 14 to execute various functions, data used by the programs, and the like.

[0038] The storage unit 19 further stores a database 20 and a phase fraction prediction model 21. The database 20 stores various data to be input to the control device 14. Examples of the data to be input to the database 20 include operational performance data of the continuous annealing equipment 1, and the phase fraction, decarburized layer thickness, and decarburization amount of the steel sheet S manufactured in the continuous annealing equipment 1. Other examples of the data to be input to the database 20 include performance data on the iron oxide layer thickness, performance data on the dew point of the soaking zone 7, and performance data on the humidifying gas flow rate.

[0039] Furthermore, a phase fraction prediction model 21 created in advance by the phase fraction prediction model generation unit 18 is stored in the storage unit 19. The phase fraction prediction model 21 may be a trained machine learning model trained by machine learning. The generation of the phase fraction prediction model 21 will be described later.

[0040] (Humidification control unit) The humidification control unit 15 calculates the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S, for example, from the thickness of the decarburized layer and the thickness of the internal oxidation layer of the steel sheet S, and measures or calculates the amount of moisture contained in the exhaust gas discharged from the soaking zone 7. The humidification control unit 15 also calculates the difference between the amount of moisture contained in the exhaust gas and the amount of moisture contained in the humidified gas supplied to the soaking zone 7. The humidification control unit 15 then compares this difference with the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S, and controls the humidifier 8 based on the result to adjust the flow rate of the humidified gas supplied to the soaking zone 7, thereby changing the dew point of the soaking zone 7. For example, the flow rate of the humidified gas is controlled by adjusting the aperture of a solenoid valve (not shown) installed in the humidified gas flow path using the control device 14 based on a signal from the humidification control unit 15. The aperture of the solenoid valve is determined by PID control using the flow rate of the humidified gas as a control variable.

[0041] The amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S described above can be calculated, for example, by determining the carbon diffusion rate and the decarburization reaction rate on the steel sheet surface from the predicted phase fraction. Phase fraction prediction methods include the use of the phase fraction prediction model described below, as well as offline predictions that take into account the crystal structure of each phase, such as the phase field method. Alternatively, the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S can be determined experimentally, and a prediction formula for the amount of moisture consumed in the steel sheet S can be created based on the moisture amount and experimental data on the composition of the steel sheet S. The amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S can also be calculated based on this prediction formula. Furthermore, the phase fraction of the steel sheet S annealed in the continuous annealing equipment 1 can be predicted using the phase fraction prediction model 21 described above, and the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S can be determined based on the phase fraction. The phase fraction of steel sheet S predicted by phase fraction prediction model 21 is the phase fraction of steel sheet S at the maximum temperature reached in rapid heating zone 9. The amount of moisture consumed in the above-mentioned decarburization and internal oxidation of steel sheet S, that is, the amount of moisture supplied to soaking zone 7 to control the dew point, also varies depending on the component amounts of steel sheet S. For this reason, it is preferable to understand the response characteristics of the amount of decarburization to changes in dew point for each steel type through prior experiments and numerical analysis and create a model formula. In actual operation, it is preferable to accumulate data showing the relationship between dew point and amount of decarburization, and adjust the amount of moisture supplied to soaking zone 7 based on the model formula while fine-tuning the parameters in the model formula as needed.

[0042] The humidification control unit 15 calculates the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S, and also calculates the amount of exhaust gas in the soaking zone 7 and the amount of moisture contained in the exhaust gas, as described above. The amount of moisture contained in the exhaust gas can be calculated, for example, by measuring the temperature and humidity of the exhaust gas. The difference between the amount of moisture contained in the exhaust gas and the amount of moisture contained in the humidified gas supplied to the soaking zone 7 is then calculated. This difference is compared with the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S, and the flow rate of the humidified gas required to maintain the dew point in the soaking zone 7 is controlled based on the result. For example, if the calculated amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S is greater than the difference, there is a possibility that the amount of moisture required for decarburization is insufficient. Therefore, in such a case, the humidification control unit 15 controls the humidifier 8 to increase the flow rate of the humidified gas. To improve the responsiveness of dew point control in the soaking zone 7, it is preferable to control the dew point of the humidified gas.

[0043] (rapid heating control unit) The rapid heating control unit 16 controls the frequency of the high-frequency current applied to the induction heating device 9A in the rapid heating zone 9 and the output of the induction heating device 9A. When the continuous annealing equipment 1 starts operation, the frequency of the high-frequency current applied to the induction heating device 9A 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 9A indicates the heating rate of the steel sheet S by the induction heating device 9A. The output of the induction heating device 9A is determined based on the size of the steel sheet S, 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 7 (i.e., the temperature of the steel sheet S at the inlet side of the induction heating device 9A), and the target annealing temperature of the steel sheet S. In this way, when heating of the steel sheet S begins in the induction heating device 9A in the rapid heating zone 9, the rapid heating control unit 16 feedforward controls the frequency of the high-frequency current applied to the induction heating device 9A based on the decarburized layer thickness of the steel sheet S. Furthermore, the rapid heating control unit 16 feedback-controls the output of the rapid heating device based on the phase fraction of the steel sheet S. The above-mentioned decarburized layer thickness may be either an actually measured value or an estimated value, and the phase fraction of the steel sheet S may be either an actually measured value or an estimated value. The frequency of the high-frequency current applied to the induction heating device 9A in the rapid heating zone 9 is preferably 100 to 1000 kHz. This is because a frequency exceeding 1000 kHz increases the equipment cost of the power supply, and a frequency below 100 kHz may reduce the heating efficiency of the steel sheet S.

[0044] (Decarburized layer thickness calculation section) The decarburized layer thickness calculation unit 17 calculates the difference between the carbon amount at the inlet side of the soaking zone 7 and the carbon amount at the outlet side of the soaking zone 7, which are measured by the X-ray fluorescence analyzer 12. Then, the decarburized layer thickness on the surface of the steel sheet S in the soaking zone 7 is estimated based on the difference in carbon amount, the dew point of the soaking zone 7, the composition of the steel sheet S, and the thickness of the iron oxide layer on the steel sheet S. Alternatively, the decarburized layer thickness on the surface of the steel sheet S in the soaking zone 7 is estimated from the phase fraction, dew point, composition of the steel sheet S, and the thickness of the iron oxide layer, which are measured by the X-ray diffraction device.

[0045] (Phase fraction prediction model generation unit) The phase fraction of the steel sheet S may be calculated using a phase fraction prediction model 21 generated by a phase fraction prediction model generation unit 18. The phase fraction prediction model 21 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 21 is generated by the phase fraction prediction model generation unit 18. The phase fraction prediction model generation unit 18 prepares a plurality of training data (training data) from various performance data stored in a database 20. 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 21 is then generated by machine learning using the plurality of training data.

[0046] (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.

[0047] (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 21, 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 21.

[0048] In addition to the conveying speed of the steel sheet S, the operational conditions used as input data for the phase fraction prediction model 21 can include, for example, the pass schedule in 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 gas input rate 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 21, 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.

[0049] (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 21, 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 21. A thermometer (not shown) is installed at each of the above-mentioned temperature measurement points.

[0050] 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 21 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 the furnace temperatures at multiple locations in the heating zone 5 and soaking zone 7. 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. 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 finishing exit of 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.

[0051] 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 21. 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.

[0052] Since the phase fraction predicted by the phase fraction prediction model 21 is the phase fraction at the maximum temperature reached in the rapid heating zone 9, 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.

[0053] (Actions and Effects) According to this embodiment, the dew point in the soaking zone 7 is controlled to fall within a range of 5 to 30°C, thereby sufficiently promoting decarburization on the surface of the steel sheet S and forming a ferrite phase on the surface of the steel sheet S. The dew point is controlled based on the amount of decarburization and the thickness of the decarburized layer of the steel sheet S in the soaking zone 7. For example, the dew point is lowered when the amount of decarburization is greater than the threshold value for the decarburization amount and the thickness of the decarburized layer is greater than the threshold value for the decarburization layer thickness. The dew point is raised when the amount of decarburization is less than the threshold value for the decarburization amount and the thickness of the decarburized layer is thinner than the threshold value for the decarburization layer thickness. Therefore, even when an induction heating device 9A is installed in the rapid heating zone 9 as a rapid heating device, the steel sheet S can be heated because a ferrite phase has been formed on the surface of the steel sheet S. Furthermore, the entire steel sheet S can be rapidly heated to the target temperature by heat conduction from the surface to the interior of the steel sheet. Furthermore, in this embodiment, the frequency of the high-frequency current applied to the induction heating device 9A in the rapid heating zone 9 is controlled based on information about the decarburization layer thickness. For example, when the decarburization layer thickness is thinner than a predetermined threshold, the frequency is increased. This allows only the thin ferrite phase to be heated appropriately, improving heating efficiency.

[0054] Furthermore, the output of the induction heating device 9A in the rapid heating zone 9 is controlled based on the phase fraction of the steel sheet S measured by a transformation ratio meter 13 provided on the outlet side of the rapid heating zone 9. For example, if the austenite phase fraction is lower than a predetermined threshold, the output of the induction heating device 9A is increased. This allows the steel sheet S to be heated to the target annealing temperature more quickly and improves heating efficiency.

[0055] Furthermore, in this embodiment, the steel sheet S is rapidly heated by the induction heating device 9A. In the induction heating device 9A, when a high-frequency current is applied to the heating coil, an alternating magnetic flux is generated along the conveyance direction of the steel sheet S. Then, an induced current is generated in a direction perpendicular to the alternating magnetic flux, i.e., along the width direction of the steel sheet S, and the steel sheet S is heated by Joule heat generated at that time. Therefore, it is possible to suppress variations in heating of the steel sheet S in the width direction, and thereby suppress variations in mechanical properties in the width direction. As a result, according to this embodiment, the steel sheet S can be rapidly heated and can be heated approximately uniformly in the width direction and over the entire length of the steel sheet S, thereby producing a steel sheet S with target mechanical properties. The method of annealing a steel sheet in the continuous annealing equipment 1 of this embodiment described above corresponds to the continuous annealing method of this embodiment. Furthermore, the process of annealing a steel sheet in the continuous annealing equipment 1 of this embodiment and galvanizing it with the galvanized strip 11 to produce a galvanized steel sheet corresponds to the method of producing a galvanized steel sheet of this embodiment. [Example]

[0056] 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 determine whether the target mechanical properties could be obtained. The steel sheet described above was in the form of a coil, and was unwound by a payoff reel and supplied to the continuous annealing line.

[0057] 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 is composed of Fe and unavoidable impurities.

[0058] 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.

[0059] [Table 1]

[0060] Each slab was hot-rolled and pickled by a conventional method, and annealed and cold-rolled as necessary. Subsequently, heat treatment was performed using a conventionally known galvanized steel sheet manufacturing facility or a manufacturing facility configured substantially similarly to the manufacturing facility of this embodiment, followed by cooling and galvanization. The sheet threading speed was 40 to 120 mpm. JIS No. 5 tensile test specimens were taken from multiple locations on the final galvanized 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 galvanized steel sheet was unwound from this coil. Test specimens were taken from the center of the galvanized 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.

[0061] 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.

[0062] Example 1 Example 1 is an example in which a galvanized steel sheet was produced using a manufacturing facility configured similarly to the manufacturing facility of this embodiment shown in FIG. 1. That is, a humidifier was provided in the soaking zone, and a solenoid-type induction heater was provided on the outlet side of the soaking zone. In addition, the phase fraction and decarburized layer thickness were predicted from the steel sheet composition and the operating conditions of the manufacturing facility, and the humidifier and solenoid-type induction heater were manually controlled. As a result, the mechanical properties of all the steel sheets were within the acceptable range.

[0063] 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.

[0064] Example 3 Example 3 is an example in which decarburized layer thickness information was acquired and the humidifier was automatically controlled based on the acquired decarburized layer information. 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.

[0065] (Comparative Example 1) Comparative Example 1 is an example in which a galvanized steel sheet was produced using conventional manufacturing equipment. That is, in this example, a solenoid-type induction heating device that functions as a rapid heating zone was not provided in the continuous annealing equipment. In addition, 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.

[0066] (Comparative Example 2) Comparative Example 2 is an example in which a humidifier was installed in the soaking zone of Comparative Example 1 to control the dew point in the soaking zone. Even if a ferrite phase is formed on the surface of the steel sheet due to dew point control, the ferrite phase does not affect the heating characteristics of the radiant tube method. This is because heating using radiant tubes is performed by heat radiation from the tube surface, so the heating characteristics do not change depending on the phase of the steel sheet surface. In other words, when heating using the radiant tube method, the effect of dew point control is unlikely to be achieved. As a result, similar to Comparative Example 1, the test was partially rejected.

[0067] (Comparative Example 3) Comparative Example 3 is an example in which a solenoid induction heater 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 heater. The solenoid induction heater was controlled based on the phase fraction measured by the transformation rate meter. The solenoid induction heater significantly reduced the heating efficiency for the 1180 MPa class steel sheet and the 1470 MPa class steel sheet, which have high austenite fractions. Furthermore, dew point control was not performed in the soaking zone. As a result, the 1180 MPa class steel sheet and the 1470 MPa class steel sheet could not be heated to the target cooling start temperature, resulting in rejection.

[0068] 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]

[0069] 1. Continuous annealing equipment 2 Payoff Reel 3 Welder 4 Looper 5 Heating Zone 6 Pre-tropical zone 7. Equal Temperature 8 Humidifier 9 Rapid heating zone 9A solenoid type induction heater 10 Cooling Zone 10A First cooling zone 10B Second cooling zone 11 Plated strip 12 X-ray fluorescence analyzer (decarburization layer thickness information acquisition device) 13 Metamorphosis Rate Meter 14 Control device 15 Humidification control section 16 Rapid heating control section 17 Decarburization layer thickness calculation section 18 Phase fraction prediction model generation section 19 Memory section 20 databases 21 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 humidifier that humidifies the inside of the soaking zone; a solenoid type induction heating device provided in the rapid heating zone; a control device that controls the humidifier to set the dew point inside the soaking zone within a range of 5 to 30°C; Continuous annealing equipment for steel plates.

2. The method further comprises a transformation rate meter for measuring a phase fraction of the steel sheet, the controller further controls the rapid heating zone based on the phase fraction; 2. The continuous annealing equipment for steel sheets according to claim 1, wherein the transformation rate meter is provided between the rapid heating zone and the cooling zone in the conveying direction.

3. a decarburization layer thickness information acquisition device that acquires decarburization layer thickness information of the steel sheet located between the soaking zone and the rapid heating zone in the conveying direction, The continuous annealing facility for steel sheets according to claim 1 or 2, wherein the control device controls the humidifying device based on the decarburized layer thickness information.

4. a decarburization layer thickness information acquisition device that acquires decarburization layer thickness information of the steel sheet located between the soaking zone and the rapid heating zone in the conveying direction, The continuous annealing facility for steel sheets according to claim 2 , wherein the control device controls the rapid heating zone and the humidifying device based on the information on the decarburized layer thickness.

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

6. 4. The continuous annealing equipment for steel sheets according to claim 3, further comprising a coating zone for coating the steel sheets downstream of the cooling zone in the conveying direction.

7. 5. The continuous annealing equipment for steel sheets according to claim 4, further comprising a coating zone for coating the steel sheets downstream of the cooling zone in the conveying direction.

8. 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, In the soaking step, the dew point around the steel sheet is controlled within a range of 5 to 30°C, In the rapid heating step, the steel sheet is heated by a solenoid-type induction heating device.

9. 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 8, wherein the solenoid type induction heating device is controlled based on the phase fraction.

10. Acquire information on the decarburized layer thickness of the steel sheet at a point in time after the soaking process and before being heated in the rapid heating process; The method for continuous annealing a steel sheet according to claim 8 or 9, wherein the dew point in the soaking step is controlled based on the information on the decarburized layer thickness.

11. Acquire information on the decarburized layer thickness of the steel sheet at a point in time after the soaking process and before being heated in the rapid heating process; 10. The method for continuous annealing a steel sheet according to claim 9, wherein the solenoid type induction heating device in the rapid heating step and the dew point in the soaking step are controlled based on the decarburized layer thickness information and the phase fraction.

12. 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 claim 8 or 9; and then galvanizing the steel sheet to produce a galvanized steel sheet.

13. 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 claim 10; and then galvanizing the steel sheet to produce a galvanized steel sheet.

14. 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 claim 11; and then galvanizing the steel sheet to produce a galvanized steel sheet.

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