Method for flattening and annealing grain-oriented electrical steel sheets and method for manufacturing grain-oriented electrical steel sheets

Abstract

The objective is to propose a planar annealing method for grain-oriented electrical steel sheets and a method for manufacturing grain-oriented electrical steel sheets that suppress slip between grain-oriented electrical steel sheets and hearth rolls during planar annealing. The planar annealing method for grain-oriented electrical steel sheets is characterized in that at least one ceramic hearth roll is arranged in the annealing furnace as a hearth roll for conveying the grain-oriented electrical steel sheets, and the optimal advance rate of the ceramic hearth roll is determined by subtracting the thermal expansion rate of the hearth roll from the thermal expansion rate of the steel sheet, or by subtracting the thermal expansion rate of the hearth roll from the thermal expansion rate of the steel sheet and adding the drive Droop reduction rate. The method for manufacturing grain-oriented electrical steel sheets includes a planar annealing step in which the optimal advance rate of the hearth roll is controlled so that the slip rate between the hearth roll and the grain-oriented electrical steel sheet is below a predetermined threshold.

Description

Technical Field

[0001] The present invention relates to a method for flattening annealing of a grain-oriented electrical steel sheet used as a core material of a transformer and a method for manufacturing a grain-oriented electrical steel sheet using the method. In the specification, "normal temperature" refers to a temperature range of 5°C or higher and 35°C or lower of the standard temperature of 20°C ± 15°C defined in JIS Z8703:1983.

Background Art

[0002] In manufacturing a grain-oriented electrical steel sheet, the steel sheet is subjected to a finishing annealing at a high temperature of 1000°C or higher for a long time in a coiled state. The purpose of the finishing annealing is to sufficiently develop secondary recrystallized grains of the Goss orientation, remove impurities in the steel, or form a forsterite film on the surface of the electrical steel sheet. However, in this finishing annealing process, shape defects such as warpage in the longitudinal direction of the coil of the grain-oriented electrical steel sheet, elongation in the middle, and lateral waves occur. In order to correct such shape defects, usually, while unwinding the coil, the steel sheet is passed through a continuous annealing furnace, and continuous flattening annealing is performed while applying an appropriate tension to the grain-oriented electrical steel sheet.

[0003] Therefore, in order to solve these problems, various flattening annealing technologies have been proposed so far. For example, for the purpose of applying the above appropriate tension, various control technologies in a continuous annealing furnace have been proposed.

[0004] Patent Document 1 discloses that in a horizontal continuous heat treatment furnace for electrical steel sheets, in a high-temperature section where the furnace temperature is 800°C or higher, in order to reduce the number of installed hearth rolls, the diameter and arrangement pitch of the hearth rolls are specified. Specifically, the diameter of the hearth roll provided in the zone for heat-treating the annealed material at a furnace temperature of 800°C or higher is 150 to 300 mm, and the arrangement pitch is more than 1500 mm and 4500 mm or less. In addition, a drive motor for driving the hearth roll via a joint for each hearth roll and a drive control device for the drive motor are provided.

[0005] Furthermore, in Patent Document 1, the drive control device is configured to calculate a rotational speed for each hearth roll, with hearth roll diameter correction added to a hearth roll reference speed command corresponding to the line speed, and to control the rotational speed for each hearth roll.

[0006] Patent Document 2 discloses a method for adequately performing continuous planar annealing of electrical steel sheets by applying appropriate tension to the electrical steel sheet in a specific temperature range using a conventional hearth roll. Specifically, tension is applied to the electrical steel sheet by a dancer roll, while the current of the hearth roll's drive motor is controlled in the zone where the temperature of the electrical steel sheet is 700°C or higher. As a result, the tension applied to the electrical steel sheet in the temperature range of 700°C or higher is 0.60 to 0.80 kg / mm². 2 It is disclosed that sufficient continuous planarization annealing is performed. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 11-286731 [Patent Document 2] Japanese Patent Application Publication No. 5-209226 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the prior art disclosed in the above-mentioned patent document has the following problems. In the technology disclosed in Patent Document 1, the rotational speed of the hearth roll during general planar annealing is controlled by calculating the rotational speed by adding a hearth roll diameter correction to the line speed. However, the expansion of the steel sheet and roll due to heat is not taken into consideration. The speed difference between the steel sheet speed and the roll speed in the annealing furnace becomes large, which can lead to scratches caused by the roll and, consequently, deterioration of the interlayer resistance. Figure 1 shows an example of scratches N on the surface of the steel sheet.

[0009] In the technology disclosed in Patent Document 2, a predetermined tension is applied to the steel sheet by controlling the current of the hearth roll drive motor. This is a so-called torque control method and does not take into account the expansion of the steel sheet and roll due to heat. The speed difference between the steel sheet speed and the roll speed in the annealing furnace becomes large, which can lead to scratches caused by the roll and, consequently, deterioration of the interlayer resistance.

[0010] This invention has been made in view of the above problems, and aims to propose a method for flattening and annealing grain-oriented electrical steel sheets that suppresses slippage between the grain-oriented electrical steel sheet and the hearth roll during flattening and annealing in order to solve the above problems. Furthermore, it aims to propose a method for manufacturing grain-oriented electrical steel sheets using this flattening and annealing method. [Means for solving the problem]

[0011] The planar annealing method for grain-oriented electrical steel sheets according to the present invention, which advantageously solves the above problems, is configured as follows.

[0012] [1] A method for planarizing and annealing grain-oriented electrical steel sheets, wherein at least one ceramic hearth roll is arranged in the annealing furnace as a hearth roll for conveying the grain-oriented electrical steel sheets, and the optimal advance rate of the ceramic hearth roll is determined by subtracting the hearth roll thermal expansion rate from the steel sheet thermal expansion rate. Here, the thermal elongation of the steel sheet is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the room temperature by the coefficient of linear expansion of the steel sheet, and the thermal elongation of the hearth roll is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the room temperature by the coefficient of linear expansion of the hearth roll. [2] A method for planarizing and annealing grain-oriented electrical steel sheets, wherein at least one ceramic hearth roll is arranged in the annealing furnace as a hearth roll for conveying the grain-oriented electrical steel sheets, and the optimal advance rate of the ceramic hearth roll is determined by subtracting the thermal elongation rate of the hearth roll from the thermal elongation rate of the steel sheet and adding the drive droop reduction rate. Here, the thermal expansion coefficient of the steel sheet is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the ambient temperature by the coefficient of linear expansion of the steel sheet; the thermal expansion coefficient of the hearth roll is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the ambient temperature by the coefficient of linear expansion of the hearth roll; and the drive droop reduction ratio is a value that corrects the discrepancy between the set speed and the actual speed set in each linked drive system. [3] A planar annealing method for grain-oriented electrical steel sheets, wherein the hearth roll and the slip ratio between the hearth roll and the grain-oriented electrical steel sheet are controlled to an optimal advance rate of the hearth roll so that the slip ratio between the hearth roll and the grain-oriented electrical steel sheet is below a predetermined threshold, as described in [1] or [2] above.

[0013] The method for manufacturing grain-oriented electrical steel sheets according to the present invention, which advantageously solves the above problems, is configured as follows. [4] A method for manufacturing grain-oriented electrical steel sheets, comprising the planar annealing method for grain-oriented electrical steel sheets described in [3] above as a planar annealing step. [Effects of the Invention]

[0014] According to the present invention, in planarization annealing, slippage between the grain-oriented electrical steel sheet and the hearth roll can be suppressed by controlling the optimal advance ratio, taking into account the thermal elongation rates of the steel sheet and the hearth roll. This makes it possible to suppress damage to the steel sheet surface caused by the roll. Furthermore, in terms of quality characteristics, deterioration of the interlayer resistance of the insulating coating on the steel sheet surface can be suppressed. [Brief explanation of the drawing]

[0015] [Figure 1] This is a scanning electron microscope image showing an example of a scratch N appearing on the surface of a steel plate. [Modes for carrying out the invention]

[0016] The following describes a planarization annealing method for grain-oriented electrical steel sheets according to an embodiment of the present invention. In a method for flattening annealing of a grain-oriented electromagnetic steel sheet according to an embodiment, at least one or more ceramic hearth rolls are arranged as hearth rolls for conveying the grain-oriented electromagnetic steel sheet in an annealing furnace. Then, in order to suppress slip between the grain-oriented electromagnetic steel sheet and the hearth roll, the optimum advancement rate in the ceramic hearth roll is controlled according to the following formula (1) in which the hearth roll thermal elongation rate is subtracted from the steel sheet thermal elongation rate. Optimum advancement rate = Steel sheet thermal elongation rate - Hearth roll thermal elongation rate ··· (1)

[0017] In addition, in a method for flattening annealing of a grain-oriented electromagnetic steel sheet according to another embodiment, at least one or more ceramic hearth rolls are arranged as hearth rolls for conveying the grain-oriented electromagnetic steel sheet in an annealing furnace. Then, in order to suppress slip between the grain-oriented electromagnetic steel sheet and the hearth roll, the optimum advancement rate in the ceramic hearth roll is controlled according to the following formula (2) in which the hearth roll thermal elongation rate is subtracted from the steel sheet thermal elongation rate and the drive Droop deceleration rate is added. Optimum advancement rate = Steel sheet thermal elongation rate - Hearth roll thermal elongation rate + Drive Droop deceleration rate ··· (2)

[0018] <Optimum advancement rate> The advancement rate setting of the roll used in this embodiment is defined by the following formula (3). The advancement rate of the roll represents the degree to which the rotational speed of the roll is greater than the sheet passing speed as a ratio to the sheet passing speed. Advancement rate setting = (Roll rotational speed command / Sheet passing speed command) - 1 ··· (3) Here, for the roll rotational speed command, the peripheral speed of the roll is used, which is the product of the number of revolutions per unit time, the roll diameter, and the pi. The method for deriving the advancement rate setting according to the optimum advancement rate setting formulas shown in formulas (1) and (2) used in this embodiment will be described. [[ID=(21)]] Optimum advancement rate = Steel sheet thermal elongation rate - Hearth roll thermal elongation rate ··· (1) Optimum advancement rate = Steel sheet thermal elongation rate - Hearth roll thermal elongation rate + Drive Droop deceleration rate ··· (2) Here, the thermal expansion coefficient of the steel sheet is the value obtained by multiplying the difference between the furnace temperature and room temperature in each section of the annealing furnace by the coefficient of linear expansion of the steel sheet, and represents the thermal expansion coefficient of the steel sheet in each section. The hearth roll thermal expansion rate is the value obtained by multiplying the difference between the furnace temperature and room temperature in each section of the annealing furnace by the coefficient of linear expansion of the hearth roll, and represents the thermal expansion rate of the hearth roll in each section. The drive Droop reduction ratio related to Droop control correction is a value that corrects the discrepancy between the set speed and the actual speed set for each linked drive system. Droop control, as referred to here, is expected to improve control stability during load fluctuations in linked drive systems, for example, drive systems with multiple drives controlled by a single inverter. However, in reality, a discrepancy occurs between the command and the actual speed, so in this embodiment, this is corrected by incorporating it as a component of the optimal advance rate.

[0019] The optimal advance rate is set based on equation (1) or (2). The roll rotation speed command and sheet feeding speed command in equation (3) are determined so that the set advance rate becomes the optimal advance rate. The optimal advance rate is controlled by the determined values ​​based on the roll rotation speed command and sheet feeding speed command so as to suppress slip between the grain-oriented electrical steel sheet and the hearth roll. When applying the optimal advance rate obtained from equations (1) or (2) above to the control of setting the advance rate, it is preferable to control the slip rate within a predetermined threshold range. For example, an error of about ±5% in the slip rate is acceptable. Here, the slip rate is the percentage obtained by dividing the speed difference obtained by subtracting the sheet feeding speed from the peripheral speed of the roll by the sheet feeding speed of the steel sheet. In the above calculation, the sheet feeding speed of the steel sheet is based on the length of the steel sheet at room temperature, and the peripheral speed of the roll is calculated by multiplying the number of rotations per unit time by the roll diameter and pi, and is based on the roll diameter at room temperature.

[0020] <Hearthroll> The annealing furnace is equipped with hearth rolls for supporting and conveying steel sheets. In this embodiment, the optimal advance rate can be set by equations (1) and (2). Therefore, at least one roll with high hardness and a high coefficient of friction, such as a ceramic hearth roll, is introduced compared to carbon rolls or iron rolls. In this case, it becomes possible to suppress the slip rate to ±5% or less. Carbon rolls have a hardness HS: 50 or higher and a coefficient of friction μ = 0.30 to 0.40. Iron rolls have a hardness HS: 20 or higher and a coefficient of friction μ = 0.40 to 0.50. Ceramic rolls have a hardness HS: 70 or higher and a coefficient of friction μ = 0.60 to 0.80. Therefore, when applied to a planarization annealing furnace for grain-oriented electrical steel sheets, coating damage due to slip between the steel sheet and the roll surface can be significantly reduced. Also, since these rolls are used during planarization annealing, the applicable furnace temperature range is approximately 500°C to 950°C. Therefore, at least one ceramic hearth roll is provided as the hearth roll. Here, it is preferable to use a full ceramic hearth roll as the ceramic roll. A full ceramic hearth roll is a hearth roll in which the shaft and the roll body are all made of ceramic. Full ceramic hearth rolls are less prone to deformation due to high-temperature creep, so even when rotated at high speed, the roll is less likely to become eccentric, allowing for stable high-speed conveying of steel plates. [Examples]

[0021] Embodiments of the present invention will be further described by reference to examples. It should be noted that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. Within the scope of the present invention, the desired performance can be achieved by the embodiments.

[0022] In the manufacturing of grain-oriented electrical steel sheets, planar annealing was performed after hot rolling, cold rolling, primary recrystallization annealing, and finish annealing after application of an annealing separation agent, according to the requirements for various advance rate settings shown in Table 1. Planar annealing was performed at 800°C for 20 seconds, and 50% of the hearth rolls in the annealing furnace were full ceramic hearth rolls. The thickness of the grain-oriented electrical steel sheets passed through was 0.23 mm. The interlayer resistance value is an indicator of the characteristics of the insulating coating of the grain-oriented electrical steel sheet after planar annealing, and is expressed as the electrical resistance per unit area of ​​one sheet. The evaluation of the interlayer resistance value was 20 Ω·cm. 2 If the number of sheets is less than 20Ω·cm, it will be considered a failure and marked with "×". 2 A minimum of one sheet per sheet is considered acceptable, marked with "〇", and the resistance is 200Ω·cm. 2 A quantity of at least [number] sheets is considered to meet the measurement limit of the measuring instrument and is marked with "◎".

[0023] [Table 1]

[0024] For steel plates No. 1, 2, and 3, which did not use the optimal advance rate in equation (1), the slip rate remained between 7% and 9%, and the interlaminar resistance value was unacceptable. For steel plates No. 4 and 5, which had the optimal advance rate correction applied in equation (1), the slip rate remained between 6% and 7%, and the interlaminar resistance value was acceptable. Furthermore, for steel plates No. 6 and 7, which had the optimal advance rate correction applied in equation (2), the interlaminar resistance value was acceptable. This reached the upper limit of the measurement equipment, demonstrating the usefulness of equation (2) with added droop control correction.

[0025] In planarization annealing, slippage between the grain-oriented electrical steel sheet and the hearth roll can be suppressed by controlling the advance rate using an optimal ratio that takes into account the thermal elongation rates of the steel sheet and the hearth roll. As a result, damage to the steel sheet surface caused by the roll can be suppressed. Furthermore, in terms of quality characteristics, deterioration of the interlayer resistance of the insulating coating on the steel sheet surface can be suppressed.

[0026] Even when installing rolls with different coefficients of thermal expansion as hearth rolls, the optimal advance rate can be calculated by applying the above embodiment to each roll.

[0027] Furthermore, since the present invention makes it possible to apply ceramic hearth rolls inside an annealing furnace, the effect of the ceramic hearth rolls is the same as that of corrosion-resistant rolls coated with thermal spray. Therefore, the deterioration of interlayer resistance can be suppressed. In addition, since the ceramic hearth rolls of the present invention have corrosion resistance to the insulating coating components, damage to the insulating coating and subsequent deterioration of interlayer resistance caused by the transport of steel plates inside the annealing furnace can be suppressed. [Explanation of symbols]

[0028] N Scratch

Claims

1. A method for planarizing and annealing grain-oriented electrical steel sheets, wherein at least one ceramic hearth roll is arranged in the annealing furnace as a hearth roll for conveying the grain-oriented electrical steel sheets, and the optimal advance rate of the ceramic hearth roll is controlled. A method for flattening and annealing grain-oriented electrical steel sheets, wherein the optimal advancement ratio is determined by subtracting the hearth roll thermal elongation ratio from the steel sheet thermal elongation ratio. Here, the thermal elongation of the steel sheet is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the room temperature by the coefficient of linear expansion of the steel sheet, and the thermal elongation of the hearth roll is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the room temperature by the coefficient of linear expansion of the hearth roll.

2. A method for planarizing and annealing grain-oriented electrical steel sheets, wherein at least one ceramic hearth roll is arranged in the annealing furnace as a hearth roll for conveying the grain-oriented electrical steel sheets, and the optimal advance rate of the ceramic hearth roll is controlled. A method for planarizing and annealing grain-oriented electrical steel sheets, wherein the optimal advance rate is determined by subtracting the hearth roll thermal elongation rate from the steel sheet thermal elongation rate and adding the drive drop reduction rate. Here, the thermal expansion coefficient of the steel sheet is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the ambient temperature by the coefficient of linear expansion of the steel sheet; the thermal expansion coefficient of the hearth roll is the value obtained by multiplying the difference between the furnace temperature of each section of the annealing furnace and the ambient temperature by the coefficient of linear expansion of the hearth roll; and the drive drop reduction ratio is a value that corrects the discrepancy between the set speed and the actual speed set in each linked drive system.

3. A method for planarizing and annealing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the optimal advance rate of the hearth roll is controlled so that the slip ratio between the hearth roll and the grain-oriented electrical steel sheet is below a predetermined threshold.

4. A method for manufacturing a grain-oriented electrical steel sheet, comprising the planar annealing method for grain-oriented electrical steel sheets described in claim 3 as a planar annealing step.

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