Manufacturing facility for galvanized steel sheet and method of manufacturing galvanized steel sheet

Abstract

To provide manufacturing facilities for a galvanized steel sheet that can obtain target mechanical characteristics by rapidly heating a steel plate up to target annealing temperature, and also suppress LME cracking, and a method of manufacturing the galvanized steel sheet.SOLUTION: The present invention relates to manufacturing facilities 1 for a galvanized steel plate which has a heating zone 5, a soaking zone 7, a rapid heating zone 9, a cooling zone 10, and a galvanizing zone 11 arranged in this order in a conveying direction of a steel plate S, and the manufacturing facilities has a humidifying device 8 which controls the dew point in the soaking zone 7 and a transverse type induction heating device 9A which is provided in the rapid heating zone 9.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a manufacturing facility for a galvanized steel sheet and a manufacturing method for a galvanized steel sheet. [Background technology]

[0002] To reduce the weight and improve safety of automobiles, there is a demand for higher strength in hot-dip galvanized steel sheets (hereafter simply referred to as steel sheets), which are used in automotive parts. While various methods exist for increasing the strength of steel sheets, methods that can achieve high strength without significantly impairing the formability of steel sheets include the addition of C to utilize martensite and the addition of Si to enhance solid-solution strengthening. Meanwhile, in the manufacture of automotive parts, press-formed parts are often assembled using resistance welding (spot welding). When steel sheets contain large amounts of C and Si, residual stresses are generated near the weld during resistance welding, which can cause zinc in the coating layer to melt and diffuse into the grain boundaries. This can lead to liquid metal embrittlement (LME), which can lead to intergranular cracking (LME cracking) in the steel sheet. In particular, when welding is performed with a welding electrode at an angle to the steel sheet, the residual stress increases, potentially resulting in cracking. Residual stress is thought to increase with increasing strength of steel sheets, and therefore there is concern about the occurrence of LME cracking with increasing strength of steel sheets.

[0003] Patent Document 1 discloses a method for improving the LME cracking resistance of steel containing C and Si. According to Patent Document 1, an iron oxide layer is formed on the surface of the steel sheet in the upstream stage of the annealing furnace, and then the steel sheet is reduced in a high dew point atmosphere in the subsequent reduction annealing zone. This is said to form a Si- and C-depleted layer in the surface layer through internal Si oxidation and decarburization reactions.

[0004] Non-Patent Document 1 discloses that the decarburization reaction on the surface of a steel sheet is promoted in a temperature range where the ferrite phase and the austenite phase coexist (two-phase region). On the other hand, in order to freely control the mechanical properties of the steel sheet, it is desirable to be able to control the temperature of the steel sheet within a wide temperature range, from the two-phase region to the austenite single-phase region, at the time when cooling begins at the outlet of the annealing furnace.

[0005] Patent Document 2 discloses that an induction heating device is installed between the outlet of the soaking zone (annealing furnace) and the cooling zone, and the output of the induction heating device is controlled based on the measurement results of the phase fraction at the start of cooling, thereby controlling the steel sheet temperature at the start of cooling with high precision. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2023 / 182524 [Patent Document 2] International Publication No. 2024 / 070278 [Non-patent literature]

[0007] [Non-Patent Document 1] ISIJ International, vol. 40 2000, No.6, pp. 619-623 Summary of the Invention [Problem to be solved by the invention]

[0008] In the method disclosed in Patent Document 1, the temperature appropriate for decarburization, for example, the temperature range in which the ferrite phase and the austenite phase coexist as described in Non-Patent Document 1, does not necessarily coincide with the temperature appropriate for controlling the structure of the steel sheet, making it difficult to achieve both LME resistance and mechanical properties. Furthermore, radiant tube heating furnaces have a large thermal inertia, making it difficult to change the temperature. There is still room for improvement in quickly heating a steel sheet to the target annealing temperature to produce a steel sheet with the desired mechanical properties. Patent Document 2 does not disclose any method for suppressing LME cracking, and there is still room for improvement in suppressing LME cracking. Non-Patent Document 1 does not disclose any method for controlling the mechanical properties of a steel sheet, and there is still room for improvement in controlling the mechanical properties of a steel sheet.

[0009] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a manufacturing facility for a galvanized steel sheet, which can quickly heat a steel sheet to a target annealing temperature to obtain target mechanical properties, and which can suppress LME cracking, and a manufacturing method for a galvanized steel sheet. [Means for solving the problem]

[0010] The means for solving the above problems are as follows. [1] A manufacturing facility for galvanized steel sheets, in which a heating zone, a soaking zone, a rapid heating zone, a cooling zone, and a galvanized zone are arranged in this order in the conveying direction of the steel sheet, the manufacturing facility for galvanized steel sheets having a humidifier for controlling the dew point inside the soaking zone, and a transverse induction heating device provided in the rapid heating zone. [2] The manufacturing equipment for galvanized steel sheet according to [1], further comprising: a transformation rate meter that measures a phase fraction of the steel sheet; and a control device that controls the rapid heating zone based on the phase fraction, wherein the transformation rate meter is provided between the rapid heating zone and the cooling zone in the conveying direction. [3] The facility for manufacturing a galvanized steel sheet according to [1], 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; and a control device that controls the humidifier based on the decarburization layer thickness information. [4] A method for manufacturing a galvanized steel sheet, which comprises carrying out a heating process, a soaking process, a rapid heating process, a cooling process, and a galvanizing process on a steel sheet in this order to manufacture a galvanized steel sheet, wherein the soaking process controls the dew point around the steel sheet, and the rapid heating process heats the steel sheet using a transverse induction heating device. [5] A method for manufacturing a galvanized steel sheet according to [4], wherein the phase fraction of the steel sheet heated in the rapid heating step is measured, and the transverse induction heating device is controlled based on the phase fraction. [6] The method for producing a galvanized steel sheet according to [4] or [5], 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 obtained, and the dew point in the soaking step is controlled based on the information on the decarburized layer thickness. [Effects of the Invention]

[0011] According to the present invention, it is possible to quickly heat a steel sheet to a target annealing temperature, thereby obtaining the target mechanical properties and suppressing LME cracking. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating the configuration of a manufacturing facility for a galvanized steel sheet 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] FIG. 1 is a diagram illustrating the configuration of a manufacturing facility 1 for a galvanized steel sheet according to an embodiment of the present invention (hereinafter referred to as 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 leading steel sheet S and the front end of the trailing steel sheet S are joined together by welding.

[0014] The looper 4 is a facility that ensures excess length of the steel sheet S so that the steel sheet S can be continuously threaded when the steel sheets S are joined together by the welder 3. After passing through the looper 4, the steel sheet S enters the manufacturing facility 1. The manufacturing facility 1 is arranged with a heating zone 5, a soaking zone 7, a rapid heating zone 9, a cooling zone 10, and a galvanizing zone 11, in this order.

[0015] (heating zone) A heating zone 5 is provided on the inlet side of the manufacturing 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 such a 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. The process of heating the steel sheet S in the heating zone 5 described above corresponds to the heating process in this embodiment.

[0016] (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.

[0017] (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 of the iron oxide layer formed in the heating zone 5 by controlling the dew point inside the soaking zone 7, i.e., the dew point around the steel sheet S. Here, controlling the dew point means controlling the moisture content inside the soaking zone 7. For this reason, the manufacturing equipment 1 has a humidifier 8 that supplies humidified gas into the soaking zone 7 to humidify it.

[0018] The preferred dew point range in the soaking zone 7 is an atmospheric dew point 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, a dew point of 5°C or higher is preferable. On the other hand, if the amount of moisture is excessive, the atmosphere in the manufacturing 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 galvanization properties of the steel sheet S in the galvanized strip described below. Furthermore, the rolls and furnace walls (not shown) inside the manufacturing equipment 1 may also be damaged. Therefore, a dew point of 30°C or lower is preferable. 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.

[0019] (humidifier) The humidifier 8 may be any conventional humidifier capable of controlling 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 control range of the humidified gas flow rate supplied to the soaking zone 7, making it preferable to the other two humidifiers. Regardless of which humidifier is used as the humidifier 8, it is preferable to maintain the temperature of the pipe supplying humidified gas to the soaking zone 7 above the dew point to prevent condensation in the soaking zone 7.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] (rapid heating zone) A rapid heating zone 9 is provided downstream of the soaking zone 7 in the conveying 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, it is preferable that the target cooling start temperature of the steel sheet S in the cooling zone 10, which will be described later, is in the range from the ferrite / austenite two-phase region to the austenite single-phase region. As a rapid heating device to be installed in the rapid heating zone 9, an induction heating type heating device that has high responsiveness to temperature control (sometimes referred to as thermal responsiveness) is preferred. When the steel sheet S is heated in the above-mentioned temperature range, the Curie point at which the magnetic properties of the steel sheet S change is exceeded. Therefore, in this embodiment, a transverse induction heating device (hereinafter simply referred to as an induction heating device) 9A is installed in the rapid heating zone 9 as a rapid heating device.

[0024] The induction heating device 9A may be a conventionally known device that has a pair of cores, each consisting mainly of a coiled heating wire (hereinafter referred to as the heating coil) and an iron core, with the steel sheet S passing through the gap between the spaced-apart cores. In the induction heating device 9A, when a high-frequency current is applied to the heating coil, an alternating magnetic flux is generated in the thickness direction of the steel sheet S. Then, an induced current is generated in the steel sheet S, flowing in a direction perpendicular to the alternating magnetic flux, and the steel sheet S is heated by the Joule heat generated at that time.

[0025] 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, which may result 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, which may prevent the steel sheet S from being heated 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 manufacturing 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.

[0026] 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., when the target annealing temperature is reached or immediately thereafter. However, 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 the target annealing temperature is reached, 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. Note that the process of rapidly heating the steel sheet S in the rapid heating zone 9 corresponds to the rapid heating process of this embodiment.

[0027] 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 as a two-stage structure including a first quenching 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 such 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.

[0028] (galvanized strip) A galvanizing zone 11 is provided downstream of the cooling zone 10 in the conveying direction of the steel sheet S. A galvanizing bath containing molten zinc is provided in the galvanizing zone 11. The steel sheet S that has passed through the manufacturing 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 manufactured.

[0029] In addition, an alloying treatment facility (not shown) may be provided downstream of the galvanized strip 11 in the conveying direction of the steel sheet S. The alloying treatment facility is a facility that heats the steel sheet and performs an alloying treatment. In addition, downstream of the alloying facility in the conveying direction of the steel sheet S, a holding zone, a final cooling zone, a temper rolling facility, 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 facilities may be conventionally known facilities and are not limited thereto. These facilities are provided or used depending on the quality required of the final product.

[0030] (Decarburized layer thickness information acquisition device) X-ray fluorescence analyzers 12 functioning as decarbonization amount information acquisition devices are provided on both the entry side and exit side of the soaking zone 7 in the conveying 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 14 described later, and the control device 14 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 entry side and the carbon amount on the exit 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.

[0031] 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 the control device 14, 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.

[0032] (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.

[0033] 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.

[0034] 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, a rapid heating device 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.

[0035] (Control device) The manufacturing equipment 1 has a control device 14 that controls the various devices described above. Fig. 2 is a diagram for explaining 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 configured, for example, mainly using a microcomputer, and executes a program read from the storage unit 19 to cause 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.

[0036] 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.

[0037] The memory 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 input to the database 20 include operational performance data of the manufacturing equipment 1, performance data on the phase fraction, decarburization layer thickness, decarburization amount, and iron oxide layer thickness of the steel sheet S manufactured by the manufacturing equipment 1, performance data on the dew point of the soaking zone 7, and performance data on the humidifying gas flow rate. In addition, a phase fraction prediction model 21 created in advance by the phase fraction prediction model generation unit 18 is stored in the memory unit 19. An example of the phase fraction prediction model 21 is a trained machine learning model trained by machine learning. The generation of the phase fraction prediction model 21 will be described later.

[0038] (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.

[0039] 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. In addition to using the phase fraction prediction model described below, the phase fraction can also be predicted offline, taking into account the crystal structure of each phase using techniques 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 manufacturing facility 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 the steel sheet S predicted by the phase fraction prediction model 21 is the phase fraction of the steel sheet S at the maximum temperature reached in the rapid heating zone 9. Furthermore, the amount of moisture consumed in the decarburization and internal oxidation of the steel sheet S described above, i.e., the amount of moisture supplied to the soaking zone 7 to control the dew point, also varies depending on the amounts of components in the 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 equation. In actual operation, it is preferable to accumulate data showing the relationship between the dew point and the amount of decarburization, and adjust the amount of moisture supplied to the soaking zone 7 based on this model equation while fine-tuning the parameters in the model equation as needed.

[0040] 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.

[0041] (rapid heating control unit) The rapid heating control unit 16 changes 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 starting operation of the manufacturing equipment 1, 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 manufacturing equipment 1, or the frequency of the high-frequency current 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 manufacturing 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. When heating of the steel sheet S is initiated in the induction heating device 9A of 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, and also feedback controls the output of the rapid heating device based on the phase fraction of the steel sheet S. The decarburized layer thickness described above may be either an actual measurement value or an estimated value, and the phase fraction of the steel sheet S may be either an actual measurement value or an estimated value. The frequency of the high-frequency current applied to the induction heating device 9A of the rapid heating zone 9 is preferably 100 to 1000 Hz. This is because a frequency exceeding 1000 Hz increases the equipment cost of the power supply, and a frequency below 100 Hz may reduce the heating efficiency of the steel sheet S.

[0042] (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.

[0043] (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 manufacturing equipment 1, and (c) the temperature history of the steel sheet S, and outputs data representing the phase fraction of the steel sheet S during annealing, and is generated by the phase fraction prediction model generation unit 18. The phase fraction prediction model generation unit 18 receives as input data the actual data on the operating conditions of the manufacturing equipment 1, the alloy composition and dimensions of the steel sheet S, and the temperature history of the steel sheet S in the annealing process from various performance data stored in a database 20, and prepares a plurality of training data (training data) that receive 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.

[0044] (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 manufacturing facility 1, and this thickness is information that affects the heating and cooling rates of the steel sheet S during annealing.

[0045] (b) Operating conditions of manufacturing facility 1 The operating conditions refer to various setting values ​​that are set in the manufacturing facility 1 when the steel sheet S is annealed by the manufacturing facility 1. Among 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 manufacturing facility 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.

[0046] The operating conditions used as input data for the phase fraction prediction model 21 may include, in addition to the conveying speed of the steel sheet S, at least one of the following: the rolling temperature and pass schedule in the hot rolling process, the cooling start temperature at the hot rolling run-out table, the cooling end temperature at the hot rolling run-out table, the coiling temperature of the hot rolled coil, the cold rolling pass schedule, the set furnace temperature in the manufacturing facility 1, and the amount of gas input to the burner. 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 above-mentioned operating conditions 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.

[0047] (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 manufacturing 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 surface temperature of the steel sheet S measured at the entry side of the manufacturing 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 measurement points for the surface temperature of the steel sheet S, the more accurately the transformation temperature can be determined, thereby improving the prediction accuracy of the phase fraction by the phase fraction prediction model 21. Temperature gauges (not shown) are installed at each of the above-mentioned temperature measurement points.

[0048] Here, the temperature history of the steel sheet S may be temperature data measured within various pieces of equipment constituting the manufacturing equipment 1 or at the connections between the pieces of equipment, or may be 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 manufacturing 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 the 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 may be used to generate the steel sheet temperature prediction model, with the operating conditions of the manufacturing equipment 1, the alloy composition of the steel sheet, and the cross-sectional shape of the steel sheet S as input data, and the temperatures of the steel sheet S at the above-mentioned locations within the manufacturing equipment 1 where it is difficult to measure the temperature as output data. Examples of operating conditions of the manufacturing 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. Furthermore, the input data for the steel sheet temperature prediction model may include information on upstream processes in the manufacturing facility 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, rolling conditions in the cold rolling process, including the pass schedule, and cooling conditions such as the cooling start temperature and cooling stop temperature on the finishing exit side in the hot rolling process and the coil winding temperature. The information on the upstream processes may include at least one of these pieces of information.

[0049] 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 manufacturing facility 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.

[0050] 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 the existing manufacturing equipment 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 manufacturing equipment 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.

[0051] (Actions and Effects) According to this embodiment, the dew point in the soaking zone 7 is controlled by the humidifier 8 so that the dew point is within a range of 5 to 30°C. The dew point is controlled by the humidifier 8 based on the amount of decarburization and the decarburization layer thickness 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 decarburization layer thickness 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 decarburization layer thickness is thinner than the threshold value for the decarburization layer thickness.

[0052] Therefore, according to this embodiment, the carbon and Si contents on the surface of the steel sheet S can be sufficiently reduced. Furthermore, since the induction heating device 9A, which is a transverse induction heating device, is installed in the rapid heating zone 9 as a rapid heating device, the steel sheet S can be rapidly heated even when the target cooling start temperature of the steel sheet S is in the austenite single-phase region. Furthermore, the frequency of the high-frequency current applied to the induction heating device 9A and the output of the induction heating device 9A are controlled based on the phase fraction of the steel sheet S measured by a transformation ratio meter 13 installed on the outlet side of the rapid heating zone 9. For example, when the austenite phase fraction is low, 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. The annealed steel sheet S is then galvanized in the galvanizing zone 11. As a result, according to this embodiment, a galvanized steel sheet having the desired mechanical properties can be manufactured. Furthermore, an iron oxide layer is formed on the surface of the steel sheet in the upstream stage of the annealing furnace, and the steel sheet is subsequently reduced in a high-dew-point atmosphere in the reduction annealing zone. As a result, a Si- and C-depleted layer can be formed in the surface layer by Si internal oxidation and decarburization reactions, thereby suppressing LME cracking. Note that the method for producing a galvanized steel sheet using the above-described galvanized steel sheet production equipment 1 of the present embodiment corresponds to the method for producing a galvanized steel sheet of the present embodiment. [Example]

[0053] An experiment was conducted to determine whether DP steel (a dual-phase steel, hereinafter simply referred to as steel sheet) could be annealed using manufacturing equipment configured almost identically to manufacturing equipment 1 shown in Figure 1 to produce a galvanized steel sheet with the desired mechanical properties. The steel sheet described above is in the form of a coil, and is unwound by a payoff reel and supplied to the manufacturing equipment.

[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 is composed of Fe and unavoidable 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 at multiple strength levels were manufactured to examine the variation in the mechanical properties of the products. Three strength levels (980 MPa, 1180 MPa, and 1470 MPa) were manufactured, and three thicknesses (1.0 mm, 1.5 mm, and 2.0 mm) were manufactured, with 10 products at each strength level. 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. In Table 1, TS represents tensile strength, EL represents ductility, and λ represents hole expandability.

[0056] [Table 1]

[0057] Each slab was hot-rolled and pickled by a conventional method, and annealed and cold-rolled as necessary. Then, heat treatment was performed using a conventionally known manufacturing facility for galvanized steel sheets 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 three random locations on the final product, the galvanized steel sheets, and the test specimens were used to perform a tensile test in accordance with the provisions of JIS Z 2241:2023.

[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] LME resistance was evaluated by the presence or absence of cracks when welding tests were conducted on each of the galvanized steel sheets of Examples 1 to 3 and Comparative Examples 1 to 4. That is, in Table 1, "◎" is entered when the conditions for cracking were alleviated compared to low-alloy high-tensile steel sheets with a tensile strength of 1180 MPa, "◯" is entered when the conditions were the same, and "×" is entered when the conditions were worse. "◎" and "◯" were judged to be acceptable. Note that "alleviated conditions for cracking compared to low-alloy high-tensile steel sheets with a tensile strength of 1180 MPa" means that the number of times the spot welding was performed was reduced.

[0060] Example 1 Example 1 is an example in which a galvanized steel sheet was produced using production equipment configured substantially similarly to the production equipment of this embodiment shown in Figure 1. That is, a humidifier was provided in the soaking zone, and a transverse 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 production equipment, and the humidifier and transverse induction heater were manually controlled. As a result, both the LME resistance and mechanical properties were within the acceptable range.

[0061] Example 2 Example 2 is an example in which a transformation rate meter was provided as a phase fraction information acquisition device, and the transverse 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, both the LME resistance and mechanical properties were within the acceptable range, and the variation in mechanical properties was reduced compared to Example 1.

[0062] 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, it was the same as Example 2. In Example 3, both the LME resistance and mechanical properties were within the acceptable range, and the variations in the mechanical properties and LME resistance were reduced compared to Example 1.

[0063] (Comparative Example 1) Comparative Example 1 is an example in which a galvanized steel sheet was manufactured using conventional manufacturing equipment. In Comparative Example 1, the target LME resistance was not achieved, and the mechanical properties varied greatly, resulting in some of the steel sheets being unacceptable.

[0064] (Comparative Example 2) Comparative Example 2 is an example in which a rapid heating zone was installed in a conventionally known manufacturing facility, and a solenoid-type induction heating device was installed as the rapid heating device in the rapid heating zone. Other aspects were the same as Comparative Example 1. The solenoid-type induction heating device had significantly poor heating efficiency for 1180 MPa-class steel sheets and 1470 MPa-class steel sheets, which have a high austenite phase fraction. Therefore, in Comparative Example 2, rapid heating sufficient to obtain the desired mechanical properties could not be performed for the 1180 MPa-class steel sheets and 1470 MPa-class steel sheets, and they were rejected. Furthermore, LME resistance could not be improved.

[0065] (Comparative Example 3) Comparative Example 3 is an example in which a rapid heating zone was provided in a conventionally known manufacturing facility, and a transverse induction heating device was provided as the rapid heating device in the rapid heating zone. The rest of the comparative example was the same as Comparative Example 2. In Comparative Example 3, the mechanical properties were acceptable, but the LME resistance was unacceptable.

[0066] Comparative Example 4 Comparative Example 4 is an example in which a humidifier was installed in the soaking zone of a conventional manufacturing facility and the dew point of the soaking zone was automatically controlled by the humidifier. In Comparative Example 4, the LME resistance was acceptable, but the target cooling start temperature could not be achieved, and the mechanical properties were unacceptable.

[0067] Thus, in Examples 1 to 3, in products with high EL (ductility) and high λ (hole expandability), where it is difficult to reduce C and Si in terms of component design, it was possible to suppress the deterioration of LME resistance by controlling the local components on the surface using internal oxidation and decarburization. [Explanation of symbols]

[0068] 1. Galvanized steel sheet manufacturing 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 transverse type induction heater 10 Cooling Zone 11 Plated strip 12 X-ray fluorescence analyzer (decarburized layer thickness information acquisition unit) 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 manufacturing facility for a galvanized steel sheet, comprising a heating zone, a soaking zone, a rapid heating zone, a cooling zone, and a galvanizing zone, which are arranged in this order in a conveying direction of the steel sheet, a humidifier for controlling the dew point inside the soaking zone; a transverse induction heating device provided in the rapid heating zone; Galvanized steel sheet manufacturing equipment.

2. a transformation rate meter for measuring the phase fraction of the steel sheet; a control device that controls the rapid heating zone based on the phase fraction, The facility for manufacturing a galvanized steel sheet 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 conveyance direction; The facility for manufacturing a galvanized steel sheet according to claim 1 , further comprising: a control device that controls the humidifier based on the decarburized layer thickness information.

4. A method for producing a galvanized steel sheet, which comprises subjecting a steel sheet to a heating step, a soaking step, a rapid heating step, a cooling step, and a galvanizing step in this order to produce a galvanized steel sheet, In the soaking step, a dew point around the steel sheet is controlled, In the rapid heating step, the steel sheet is heated by a transverse induction heating device.

5. measuring a phase fraction of the steel plate heated in the rapid heating step; The method for producing a galvanized steel sheet according to claim 4, wherein the transverse induction heating device is controlled based on the phase fraction.

6. 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 producing a galvanized steel sheet according to claim 4 or 5, wherein the dew point in the soaking step is controlled based on the information on the decarburized layer thickness.

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