Method for flattening annealing grain-oriented electromagnetic steel sheet and method for producing grain-oriented electromagnetic steel sheet

Abstract

The purpose of the present invention is to propose a method for flattening annealing a grain-oriented electromagnetic steel sheet and a method for producing a grain-oriented electromagnetic steel sheet in which slip between a grain-oriented electromagnetic steel sheet and hearth rolls is suppressed in flattening annealing. In this method for flattening annealing a grain-oriented electromagnetic steel sheet, one or more ceramic hearth rolls are arranged as hearth rolls for transporting the grain-oriented electromagnetic steel sheet in an annealing furnace, and when controlling an optimum forward slip rate in the ceramic hearth rolls, the optimum forward slip rate is determined by subtracting the hearth roll thermal elongation rate from the steel sheet thermal elongation rate or by subtracting the hearth roll thermal elongation rate from the steel sheet thermal elongation rate and adding a drive droop deceleration rate. The method for producing a grain-oriented electromagnetic steel sheet includes a flattening annealing step in which the optimum forward slip rate of the hearth rolls is controlled so that the slip rate between the hearth rolls and the grain-oriented electromagnetic steel sheet is equal to or less than a predetermined threshold value.

Description

Flattening annealing method for grain-oriented electrical steel sheet and manufacturing method for grain-oriented electrical steel sheet

[0001] The present invention relates to a method for flattening and annealing grain-oriented electrical steel sheets used as iron core materials for transformers, and a method for manufacturing grain-oriented electrical steel sheets using this method. In the specification, "room temperature" refers to a temperature range of 5°C to 35°C, within the standard temperature range of 20°C ± 15°C, as defined in JIS Z8703:1983.

[0002] In the production of grain-oriented electrical steel sheets, the steel sheets are wound into a coil and subjected to a long-term finish annealing process at a high temperature of 1000°C or higher. The purpose of this finish annealing is to sufficiently develop secondary recrystallized grains with a Goss orientation, to remove impurities from the steel, and to form a forsterite film on the surface of the electrical steel sheet. However, this finish annealing process can cause shape defects in the grain-oriented electrical steel sheet, such as longitudinal warpage, central elongation, and side waves. To correct these shape defects, the coil is usually passed through a continuous annealing furnace while being uncoiled, and continuous flattening annealing is performed while applying an appropriate tension to the grain-oriented electrical steel sheet.

[0003] To solve these problems, various flattening annealing techniques have been proposed. For example, various control techniques have been proposed for continuous annealing furnaces in order to provide the above-mentioned appropriate tension.

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

[0005] Furthermore, in Patent Document 1, the drive control device is configured to calculate a rotational speed for each hearth roll by adding a hearth roll diameter correction 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 sufficiently performing continuous flattening annealing of an electromagnetic steel sheet by applying an appropriate tension to the electromagnetic steel sheet in a specific temperature range using a normal hearth roll. Specifically, while applying tension to the electromagnetic steel sheet using a dancer roll, the current of the drive motor for the hearth roll is controlled in the zone where the temperature of the electromagnetic steel sheet is 700°C or higher. As a result, the tension applied to the electromagnetic steel sheet in the temperature range of 700°C or higher is set to 0.60 to 0.80 kg / mm. ２ and then performing sufficient continuous flattening annealing.

[0007] JP-A-11-286731 JP-A-5-209226

[0008] However, the conventional technology disclosed in the above-mentioned patent document has the following problems. In the technology disclosed in Patent Document 1, the rotation speed of the hearth roll during general flattening annealing is controlled by calculating the rotation speed obtained by adding a hearth roll diameter correction to the line speed. However, this does not take into consideration the elongation of the steel sheet or roll due to heat. The speed difference between the steel sheet speed and the roll speed in the annealing furnace becomes large, which may cause scratches caused by the roll and ultimately deteriorate the interlaminar resistance. Figure 1 shows an example of scratches N on the surface of a 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 so-called torque control, and does not take into consideration the elongation of the steel sheet or rolls due to heat. This increases the speed difference between the steel sheet speed and the roll speed in the annealing furnace, which can lead to scratches caused by the rolls and ultimately to a deterioration in interlaminar resistance.

[0010] The present invention has been made in view of the above problems, and aims to solve the above problems by proposing a flattening annealing method for grain-oriented electrical steel sheets that suppresses slippage between the grain-oriented electrical steel sheets and hearth rolls during flattening annealing, and a further aim to propose a method for manufacturing grain-oriented electrical steel sheets using the flattening annealing method.

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

[0012] [1] A method for flattening annealing a grain-oriented electrical steel sheet, comprising: arranging at least one ceramic hearth roll as a hearth roll for transporting the grain-oriented electrical steel sheet in an annealing furnace; and controlling an optimal forward slip for the ceramic hearth roll by subtracting the hearth roll thermal elongation from the steel sheet thermal elongation, wherein the steel sheet thermal elongation is the value obtained by multiplying the linear expansion coefficient of the steel sheet by the difference between the furnace temperature of each section of the annealing furnace and room temperature; and the hearth roll thermal elongation is the value obtained by multiplying the linear expansion coefficient of the hearth roll by the difference between the furnace temperature of each section of the annealing furnace and room temperature. [2] A method for flattening annealing a grain-oriented electrical steel sheet, comprising: an annealing furnace having at least one ceramic hearth roll disposed as a hearth roll for transporting the grain-oriented electrical steel sheet; and controlling an optimal forward slip ratio for the ceramic hearth roll by subtracting the hearth roll thermal elongation ratio from the steel sheet thermal elongation ratio and adding a drive drop deceleration ratio, wherein the steel sheet thermal elongation ratio is a value obtained by multiplying the linear expansion coefficient of the steel sheet by the difference between the furnace temperature of each section of the annealing furnace and room temperature; the hearth roll thermal elongation ratio is a value obtained by multiplying the linear expansion coefficient of the hearth roll by the difference between the furnace temperature of each section of the annealing furnace and room temperature; and the drive drop deceleration ratio is a value used to correct for the deviation between the set speed set in each connected drive system and the actual speed. [3] In the above [1] or [2], the method for flattening annealing a grain-oriented electrical steel sheet controls an optimal forward slip of the hearth roll so that the slip ratio between the hearth roll and the grain-oriented electrical steel sheet is equal to or less than a predetermined threshold value.

[0013] The method for producing a grain-oriented electrical steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is configured as follows: [4] A method for producing a grain-oriented electrical steel sheet, which includes the flattening annealing method for a grain-oriented electrical steel sheet described in [3] above as a flattening annealing step.

[0014] According to the present invention, slippage between the grain-oriented electrical steel sheet and the hearth roll can be suppressed by controlling the optimum forward slip in consideration of the thermal elongation of the steel sheet and the hearth roll during flattening annealing. This makes it possible to suppress damage to the steel sheet surface caused by the roll. Furthermore, as a quality characteristic, deterioration of the interlaminar resistance of the insulating coating on the steel sheet surface can be suppressed.

[0015] 1 is a scanning electron microscope photograph showing an example of scratches N appearing on the surface of a steel sheet.

[0016] Hereinafter, a flattening annealing method for grain-oriented electrical steel sheets according to an embodiment of the present invention will be described. In the flattening annealing method for grain-oriented electrical steel sheets according to one embodiment, at least one ceramic hearth roll is arranged as a hearth roll for transporting the grain-oriented electrical steel sheet in an annealing furnace. Then, in order to suppress slippage between the grain-oriented electrical steel sheet and the hearth roll, the optimum forward slip of the ceramic hearth roll is controlled according to the following equation (1), which subtracts the hearth roll thermal elongation from the steel sheet thermal elongation. Optimum forward slip = steel sheet thermal elongation - hearth roll thermal elongation (1)

[0017] In another embodiment of the flattening annealing method for grain-oriented electrical steel sheet, at least one ceramic hearth roll is arranged as a hearth roll for transporting the grain-oriented electrical steel sheet in an annealing furnace. The optimum forward slip of the ceramic hearth roll is controlled in accordance with the following formula (2), which subtracts the hearth roll thermal elongation rate from the steel sheet thermal elongation rate and adds the drive drop deceleration rate, so as to suppress slippage between the grain-oriented electrical steel sheet and the hearth roll: Optimum forward slip = Steel sheet thermal elongation rate - Hearth roll thermal elongation rate + Drive drop deceleration rate (2)

[0018] <Optimum forward slip ratio> The forward slip ratio setting of the roll used in this embodiment is defined by the following formula (3). The forward slip ratio of the roll represents the extent to which the rotational speed of the roll is faster than the strip threading speed, expressed as a ratio to the strip threading speed. Forward slip ratio setting = (roll rotational speed command / strip threading speed command) - 1 (3) Here, the roll rotational speed command uses the peripheral speed of the roll, and is calculated by multiplying the number of rotations per unit time by the roll diameter and pi. A method for deriving the forward slip ratio setting using the optimal forward slip ratio setting formulas shown in formulas (1) and (2) used in this embodiment will be described. Optimum forward slip ratio = steel sheet thermal elongation rate - hearth roll thermal elongation rate (1) Optimum forward slip ratio = steel sheet thermal elongation rate - hearth roll thermal elongation rate + drive drop deceleration rate (2) Here, the thermal elongation rate of the steel sheet is the value obtained by multiplying the linear expansion coefficient of the steel sheet by the difference between the furnace temperature and room temperature in each section of the annealing furnace, and is the thermal elongation rate of the steel sheet in each section. The hearth roll thermal elongation is the value obtained by multiplying the linear expansion coefficient of the hearth roll by the difference between the furnace temperature of each section of the annealing furnace and room temperature, and is the thermal elongation of the hearth roll in each section. The drive drop deceleration rate related to the drop control correction is a value that corrects the amount of deviation between the set speed set in each connected drive system and the actual speed. The drop control referred to here is expected to improve control stability during load fluctuations in connected drive systems, such as drive systems having multiple drives with one inverter. However, since deviations between command and actual values ​​actually occur in reality, in this embodiment, this is corrected as a component of the optimal forward rate.

[0019] The optimal forward slip ratio is set based on equation (1) or (2). The roll rotation speed command and the strip threading speed command are determined based on equation (3) so that the set forward slip ratio becomes the optimal forward slip ratio. The optimal forward slip ratio is controlled based on the determined values ​​based on the roll rotation speed command and the strip threading speed command so as to suppress slip between the grain-oriented electrical steel sheet and the hearth roll. When applying the optimal forward slip ratio obtained by equation (1) or (2) to the forward slip ratio setting control, it is preferable to control the slip ratio within a predetermined threshold range. For example, an error of approximately ±5% is allowed in the slip ratio. Here, the slip ratio refers to the percentage obtained by dividing the speed difference, obtained by subtracting the steel sheet threading speed from the roll peripheral speed, by the steel sheet threading speed. In the above calculation, the steel sheet threading speed is based on the length of the steel sheet at room temperature, and the roll peripheral speed 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] <Hearth Roll> The annealing furnace is equipped with a hearth roll that supports and transports the steel sheet. In this embodiment, the optimal forward slip can be set using equations (1) and (2). Therefore, at least one roll, such as a ceramic hearth roll, with higher hardness and a higher friction coefficient than carbon rolls or iron rolls is introduced. In this case, it is possible to suppress the slip ratio to ±5% or less. The carbon roll has a hardness HS of 50 or more and a friction coefficient μ of 0.30 to 0.40. The iron roll has a hardness HS of 20 or more and a friction coefficient μ of 0.40 to 0.50. The ceramic roll has a hardness HS of 70 or more and a friction coefficient μ of 0.60 to 0.80. Therefore, when applied to a flattening annealing furnace for grain-oriented electrical steel sheets, it can significantly reduce coating damage due to slip between the steel sheet and the roll surface. Furthermore, because these rolls are used during flattening annealing, the applicable furnace temperature range is approximately 500°C to 950°C. Therefore, at least one ceramic hearth roll is disposed as the hearth roll. Here, it is preferable to use a full ceramic hearth roll as the ceramic roll. The full ceramic hearth roll is a hearth roll in which the shaft portion and the roll body are all formed of ceramic. The full ceramic hearth roll is resistant to high-temperature creep deformation and therefore is less likely to become eccentric even when rotated at high speed, allowing for stable high-speed transport of steel sheets.

[0021] The embodiments of the present invention will be further explained by way of examples. Note that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. The embodiments can achieve the desired performance within the scope of the present invention.

[0022] In the production of grain-oriented electrical steel sheets, the sheets were subjected to hot rolling, cold rolling, primary recrystallization annealing, and finish annealing after application of an annealing separator, followed by flattening annealing under the various forward slip setting requirements shown in Table 1. The flattening 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 to be passed was 0.23 mm. The interlaminar resistance is an index showing the properties of the insulating coating of the grain-oriented electrical steel sheet after flattening annealing, and is expressed as the electrical resistance per unit area of ​​one sheet. The interlaminar resistance was evaluated at 20 Ω cm ２A sheet less than 20 Ω cm is rejected and marked with "x". ２ / sheet or more is considered to be pass, marked with "〇", and ２ A value of 1 / 100 or more is considered to pass the upper limit of the measurement by the measuring instrument and is marked with "◎".

[0023]

[0024] For steel plates Nos. 1, 2, and 3, which did not use the optimal forward slip ratio according to formula (1), the slip ratio fluctuated between 7 and 9%, resulting in unacceptable interlaminar resistance values. For steel plates Nos. 4 and 5, which used correction for the optimal forward slip ratio according to formula (1), the slip ratio fluctuated between 6 and 7%, resulting in acceptable interlaminar resistance values. Furthermore, for steel plates Nos. 6 and 7, which used correction for the optimal forward slip ratio according to formula (2), the interlaminar resistance values ​​also passed. These values ​​were within the upper measurement limit of the measuring equipment, demonstrating the usefulness of formula (2) with drop control correction.

[0025] In the flattening annealing, slippage between the grain-oriented electrical steel sheet and the hearth roll can be suppressed by controlling the forward slippage using an optimal forward slip ratio that takes into account the thermal elongation of the steel sheet and the hearth roll. As a result, damage to the steel sheet surface caused by the roll can be suppressed. In addition, as a quality characteristic, deterioration of the interlaminar resistance of the insulating coating on the steel sheet surface can be suppressed.

[0026] When rolls having different thermal expansion coefficients are installed as hearth rolls, the optimum forward slip can be calculated by applying the above embodiment to each roll.

[0027] Furthermore, since the present invention makes it possible to apply a ceramic hearth roll to an annealing furnace, the effect of the ceramic hearth roll is similar to that of a corrosion-resistant roll obtained by thermal spray coating. Therefore, deterioration of interlaminar resistance can be suppressed. Furthermore, since the ceramic hearth roll of the present invention is corrosion-resistant to the components of the insulating coating, damage to the insulating coating and thus deterioration of interlaminar resistance caused by transport of steel sheets in the annealing furnace can be suppressed.

[0028] N Scratch

Claims

1. A method for flattening annealing a grain-oriented electrical steel sheet, comprising: disposing at least one ceramic hearth roll as a hearth roll for transporting the grain-oriented electrical steel sheet in an annealing furnace; and controlling the optimal forward slip ratio of the ceramic hearth roll by subtracting the hearth roll thermal elongation from the steel sheet thermal elongation, wherein the steel sheet thermal elongation ratio is the value obtained by multiplying the linear expansion coefficient of the steel sheet by the difference between the furnace temperature of each section of the annealing furnace and room temperature; and the hearth roll thermal elongation ratio is the value obtained by multiplying the linear expansion coefficient of the hearth roll by the difference between the furnace temperature of each section of the annealing furnace and room temperature.

2. A method for flattening annealing grain-oriented electrical steel sheet, wherein at least one ceramic hearth roll is arranged in an annealing furnace as a hearth roll for transporting the grain-oriented electrical steel sheet, and when controlling the optimal forward slip of the ceramic hearth roll, the optimal forward slip is determined by subtracting the hearth roll thermal elongation from the steel sheet thermal elongation and adding a drive drop deceleration rate, where the steel sheet thermal elongation rate is the value obtained by multiplying the linear expansion coefficient of the steel sheet by the difference between the furnace temperature of each section of the annealing furnace and room temperature, the hearth roll thermal elongation rate is the value obtained by multiplying the linear expansion coefficient of the hearth roll by the difference between the furnace temperature of each section of the annealing furnace and room temperature, and the drive drop deceleration rate is a value used to correct for the difference between the set speed set in each connected drive system and the actual speed.

3. A method for flattening annealing grain-oriented electrical steel sheet according to claim 1 or 2, wherein the optimum forward slip of the hearth roll is controlled so that the slip ratio between the hearth roll and the grain-oriented electrical steel sheet is equal to or less than a predetermined threshold value.

4. A method for producing grain-oriented electrical steel sheet, comprising the flattening annealing method for grain-oriented electrical steel sheet according to claim 3 as a flattening annealing step.

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