Patent Application
20240392403
The present invention relates to a method for annealing a steel strip subjected to hot rolling (hot-rolled steel strip) used to produce an electrical steel sheet.
It is known that the magnetic properties of a product sheet can be improved, that is, the Goss orientation can be highly developed, by subjecting a hot-rolled steel strip containing 1.6 to 5.0 mass % Si for an electrical steel sheet to annealing (hot-band annealing) before subjecting it to cold rolling. Such hot-band annealing is typically performed using continuous annealing equipment including a heating zone, a soaking zone, and a cooling zone.
It is also known that the annealing temperature during the hot-band annealing greatly affects the magnetic properties of a product sheet and that it is necessary to control the temperature of the hot-rolled steel strip so that it is constant and uniform over the entire length and width of the steel strip. Therefore, in continuous annealing equipment, the furnace temperature in the soaking zone is usually controlled to be constant so that the heating temperature for the hot-rolled steel strip to be annealed can be controlled in a preferable range.
However, with the above-described conventional annealing method in which only the furnace temperature in the soaking zone is controlled, it is difficult to suppress the variation in the heating temperature for the steel strip due to the variation in the thickness of the steel strip in the longitudinal direction or the variation in the threading speed of the steel strip, and thus to control the heating temperature within a preferable range. This is because a hot-rolled steel strip before cold rolling is thick and thus has a high heat capacity per unit length (i.e., the amount of heat required to increase the temperature of an object by 1° C.), which requires a larger amount of heat for heating than a cold-rolled steel strip. Further, a hot-rolled steel strip is thick and thus has a large variation in thickness with respect to the rolled sheet thickness, particularly in thickness at the front end of the steel strip, and the heat capacity largely varies within the same steel strip. Therefore, compared with a cold-rolled steel strip, the hot-rolled steel strip greatly affects the temperature of an annealing furnace, and thus it is difficult to maintain the furnace temperature in each of the heating zone and the soaking zone at a predetermined temperature and thus to heat the hot-rolled steel strip to a predetermined soaking temperature.
However, there has been little disclosure of technology for controlling the heating temperature of hot-band annealing with high accuracy. For example, Patent Literature 1 discloses a technology for reducing the aging index of a steel sheet before it is subjected to the final cold rolling in the production of a grain-oriented electrical steel sheet by controlling the cooling rate of hot-band annealing.
Patent Literature 2 discloses a technology for performing hot-band annealing on a Si-containing steel sheet involving rapidly heating a steel strip during hot-band annealing.
Although Patent Literature 1 discloses a technology for improving the texture of a hot-rolled sheet for a grain-oriented electrical steel sheet by performing hot-band annealing on the sheet, it fails to mention the necessity of controlling the heating temperature of the hot-band annealing with high accuracy, or means therefor.
In addition, Patent Literature 2 employs rapid heating during hot-band annealing as a means to improve the descaling property after annealing, but does not mention precise control of the heating temperature after rapid heating.
As described above, in the production of an electrical steel sheet, precise control of the heating temperature in hot-band annealing is crucial for improving magnetic properties. However, due to the inherent characteristics of a hot-rolled steel strip, such as large plate thickness, low thickness accuracy, and large thickness variation, the temperature in the annealing furnace varies correspondingly. Therefore, there have been limitations in controlling the heating temperature of the steel strip with high precision.
An object of the present invention is to solve the above problems of the conventional technologies and to provide a method for annealing a hot-rolled steel strip that can control the steel strip temperature on the exit side of a heating zone as well as the steel strip temperature on the exit side of a soaking furnace with high accuracy during hot-band annealing.
Herein, the steel strip temperature on the exit side of the heating zone refers to the steel strip temperature at the exit of the heating zone.
The inventors have made concentrated studies to solve the above problems and have developed the present invention by finding that, to control the heating temperature of hot-band annealing with high accuracy, it is effective to arrange, in a continuous annealing furnace including a heating zone, a soaking zone, and a cooling zone, a thickness meter and a rapid heating device on the upstream side of the soaking zone, and further change the heating temperature of the rapid heating device for a steel strip in accordance with the variation in LSD (i.e., the thickness×the threading speed) of the steel strip.
That is, a method for annealing a hot-rolled steel strip for an electrical steel sheet of the present invention is configured as follows.
[1] A method for annealing a hot-rolled steel strip containing 1.6 to 5.0 mass % Si for an electrical steel sheet using annealing equipment including a heating zone, soaking zone, and cooling zone arranged in this order from an upstream side. The method includes arranging a thickness meter and a rapid heating device on an upstream side of the soaking zone and determining the heating temperature of the rapid heating device for the steel strip from a value of LSD defined by the thickness of the steel strip measured by the thickness meter and the threading speed of the steel strip during annealing as represented by the following Expression (1):
[2] In [1] above, the heating temperature of the rapid heating device for the steel strip can be determined based on, in addition to the LSD, information on at least one of the width of the steel strip, the furnace temperature in the heating zone on the subsequent stage after the rapid heating device, the steel strip temperature on the exit side of the heating zone, the furnace temperature in the soaking zone, and the steel strip temperature on the exit side of the soaking zone.
[3] In [1] or [2] above, the rapid heating device can be arranged upstream of the heating zone or in a front stage or a middle stage of the heating zone.
[4] In any one of [1] to [3] above, the heating temperature of the rapid heating device for the steel strip can be determined by feeding back a furnace temperature in the heating zone on the subsequent stage after the rapid heating device.
[5] In any one of [1] to [4] above, the heating temperature of the rapid heating device for the steel strip can be determined to maintain a constant steel strip temperature on an exit side of the heating zone.
[6] In any one of [1] to [5] above, the rapid heating device can use, as heating means, one or more of induction heating, electrical heating, and infrared heating.
The present invention can control the steel strip temperature on the exit side of the heating zone as well as the steel strip temperature on the exit side of the soaking zone to a predetermined target temperature with high accuracy, which can be achieved by heating the steel strip such that the heating temperature of a rapid heating device for the steel strip is set higher as the value of LSD defined by the thickness×the threading speed of the steel strip is greater and is set lower as the LSD is smaller. This greatly contributes to improving the magnetic properties of the resulting product sheet and reducing the occurrence of defects.
Hereinafter, the present invention will be described with reference to the drawings.
However, the steel strip inevitably has a variation in thickness and particularly has a large variation in thickness at the front and rear end portions of the steel strip. Further, there may be a case where the threading speed of the steel strip is suddenly changed while the steel strip is annealed. When the steel strip has a large variation in thickness or when the threading speed of the steel strip is suddenly changed, it is difficult to heat the steel strip to a target temperature only by controlling the furnace temperature in each of the heating zone and the soaking zone to be constant. Thus, the preset temperature of the furnace needs to be changed. However, suddenly changing the preset temperature of the furnace will cause an overshoot or hunting not only of the furnace temperature but also of the steel strip temperature. Further, it takes some time to change the furnace temperature, so that the furnace temperature cannot be changed immediately. Therefore, it is desirable to set the temperature of an annealing furnace 1 as constant as possible or to change the temperature gradually.
Further, a hot-rolled steel strip is thicker and has a larger heat capacity compared to a cold-rolled steel strip as described above, which makes it difficult to follow the change in the furnace temperature in the heating zone. Moreover, heating the steel strip to a target temperature is difficult as it can affect the furnace temperature in the heating zone 3. Therefore, in the heating zone 3, it is desired to heat the steel strip so that the steel strip temperature on the exit side of the heating zone can consistently maintain the predetermined target temperature, independently of the variation in the thickness or the threading speed of the steel strip.
The final purpose of the hot-band annealing is to perform heat treatment for holding the steel strip S at a target soaking temperature in the soaking zone 4 for a predetermined period. However, a hot-rolled steel strip, which is thicker than a cold-rolled steel strip, possesses a different heat amount when the steel strip temperature on the exit side of the heating zone is different, causing the amount of heat transferred to the soaking zone 4 to vary. This will cause the furnace temperature to vary, making it difficult to heat the hot-rolled steel strip to the target soaking temperature. In this sense, it is still preferable to set the steel strip temperature on the exit side of the rapid heating device 2 so that the steel strip temperature on the exit side of the heating zone 3 can maintain a constant value.
Thus, in the present invention, when a hot-rolled steel strip for an electrical steel sheet is subjected to hot-band annealing using the continuous annealing equipment shown in
In the present invention, when the value of LSD of the hot-rolled steel strip is larger, the heating temperature of the rapid heating device 2 for the steel strip is set higher. Conversely, when the value of LSD of the hot-rolled steel strip is smaller, the heating temperature of the rapid heating device 2 is set lower. More specifically, heating is performed such that the steel strip temperature on the exit side of the rapid heating device, that is, the heating temperature of the rapid heating device 2 is set higher than the conventional heating temperature for which the LSD is not considered as the thickness of the steel strip, including a slight variation in the thickness of the steel strip, becomes larger, while the steel strip temperature on the exit side of the rapid heating device, that is, the heating temperature of the rapid heating device 2 is set lower than the conventional heating temperature for which the LSD is not considered as the thickness of the steel strip becomes smaller. Similarly, heating is performed such that when the line speed has increased, the steel strip temperature on the exit side of the rapid heating device, that is, the heating temperature of the rapid heating device 2 is set higher than the conventional heating temperature for which the LSD is not considered, while when the line speed has decreased, the steel strip temperature on the exit side of the rapid heating device 2, that is, the heating temperature of the rapid heating device 2 is set lower than the conventional heating temperature for which the LSD is not considered.
By considering the value of LSD as described above, even when there is a variation in the thickness t of the hot-rolled steel strip S or when the threading speed LS of the hot-rolled steel strip S has suddenly changed, it is possible to suppress the variation in the steel strip temperature on the exit side of the heating zone as well as the variation in the furnace temperature in the soaking zone without changing the preset value of the furnace temperature in the heating zone 3, and thus to control the temperature of the hot-rolled steel strip to a target soaking temperature with high accuracy.
Herein, the rapid heating device 2 may be any device that can increase or change the steel strip temperature with high responsiveness. In addition, as the method for heating the steel strip, any method having many track records, such as induction heating (e.g., solenoid method and transverse method), electrical heating, or near-infrared heating, can be suitably used.
The rapid heating device 2 is arranged upstream of the soaking zone in order to control the steel strip temperature on the exit side of the heating zone and thus the steel strip temperature on the exit side of the soaking furnace with high accuracy. As shown in
The heating ability of the rapid heating device 2 is preferably greater to accommodate a larger thickness or a large variation in the threading speed of the hot-rolled steel strip. As one guide, when the heating temperature for a steel strip with a thickness of about 2 mm is to be changed by about +20° C. on the exit side of the heating zone, the rapid heating device 2 with a heating ability (the amount of temperature increase) of about 50 to 100° C. can sufficiently suppress the variation, though it differs depending on the temperature range (position) in which the rapid heating device is arranged and the “temperature-specific heat” relationship of the steel strip.
As described above, changing the preset value of the heating temperature of the rapid heating device 2 for the steel strip in accordance with the value of LSD, that is, the variation of (thickness× threading speed), can sufficiently contribute to controlling the heating temperature for a steel strip with high accuracy. However, to control the steel strip temperature on the exit side of the heating zone 3 with even higher accuracy, it is also preferable to consider the width of the hot-rolled steel strip S, in addition to the value of LSD. This is because since the heat capacity of a steel strip varies in proportion to the width of the steel strip, the amount of heat required to heat the steel strip and the amount of heat removed from the furnace body also vary correspondingly, and thus, the width of the steel strip also affects the steel strip temperature on the exit side of the heating zone and the furnace temperature in each of the heating zone and the soaking zone. In particular, when using annealing equipment that includes furnace bodies with low heat capacity and heating and soaking zones where the furnace temperature is likely to change, it is desirable to consider the width of the steel strip when setting the heating temperature of the rapid heating device.
Further, to achieve more accurate control of the steel strip temperature on the exit side of the heating zone, it is preferable to consider information on the furnace temperature in the heating zone on the subsequent stage after the rapid heating device when setting the heating temperature of the rapid heating device 2. For example, when the heating ability of the rapid heating device 2 is low, using the rapid heating device may be insufficient when the heat capacity of the steel strip has changed to a great degree due to the variation in the thickness or the threading speed of the hot-rolled steel strip or due to the variation in LSD. Thus, it is considered that changing the preset value of the furnace temperature in the heating zone may be effective.
Thus, it is possible to control the steel strip temperature on the exit side of the heating zone 3 with even higher accuracy by feeding back information on the constantly changing furnace temperature on the subsequent stage after the rapid heating device 2, and reflecting it in the preset value of the heating temperature of the rapid heating device 2.
Note that when the furnace temperature information is fed back, it is possible to calculate the amount of increase in the steel strip temperature on the exit side of the heating zone, using the following Expression (2):
With Expression (2) above, the amount of temperature increase ΔT can be adjusted by considering the variation in the furnace temperature Tf and adjusting the steel sheet temperature Ts on the entry side, that is, the heating temperature of the rapid heating device for the steel sheet, so that the steel strip temperature on the entry side of the soaking zone can be controlled. Herein, the overall heat absorption rate ϕCG is the index representing efficiency related to the heat flow throughout the entire furnace, and is determined by the actual records of the furnace temperature and sheet temperature.
To achieve more accurate control of the steel strip temperature on the exit side of the heating zone, it is preferable to also use information on the steel strip temperature on the exit side of the heating zone, furnace temperature in the soaking zone, and temperature of the steel strip on the exit side of the soaking zone to determine the heating temperature of the rapid heating device 2 for the steel strip, in addition to LSD, the steel strip width, and the furnace temperature in the heating zone on the subsequent stage after the rapid heating device described above.
Herein, the target hot-rolled steel strip for an electrical steel sheet of the present invention contains 1.6 to 5.0 mass % Si. Si is an element effective in increasing the resistivity of steel and thus reducing an iron loss and is preferably contained at 1.6 mass % or more. However, if the Si content is more than 5.0 mass %, the magnetic flux density will decrease, and the steel will become brittle, resulting in a significant deterioration in manufacturability such that cracks will occur during cold rolling. Preferably, the Si content is in the range of 2.0 to 3.8 mass %.
Note that the hot-rolled steel strip may also contain, in addition to Si, a known inhibitor-forming component to allow secondary recrystallization to occur, or a known additional element to improve magnetic or mechanical properties.
The hot-band annealing of the present invention described above allows for uniform heat treatment at an appropriate temperature throughout the entire length of the hot-rolled steel strip in the longitudinal direction (rolling direction). Thus, the final product sheet can be provided with excellent magnetic properties without causing poor secondary recrystallization or defective crystal orientations throughout the entire length of the coil in the longitudinal direction.
A hot-rolled steel strip with a thickness of 1.4 to 2.8 mm and a width of 1200 mm was subjected to hot-band annealing with a target soaking temperature of 1050° C. under the conditions described in Table 1, using the annealing furnace 1 including the rapid heating device 2, the heating zone 3, and the soaking zone 4 shown in
Note that a solenoid-type induction heating device was used as the rapid heating device 2, and the amount of change in the heating temperature of the rapid heating device for the steel strip was set to a value obtained multiplying the difference between the two values (LSDt−LSDs) by a proportional coefficient K, wherein LSDs represents the value of the preset thickness (preset rolling thickness) of the hot-rolled steel strip×the threading speed, and LSDt represents the value of the thickness of the steel strip actually measured by the thickness meter 5 arranged upstream of the rapid heating device×the threading speed. The value of K is not fixed but varies depending on the thickness of the steel strip, heating properties such as specific heat of the steel strip, arrangement position of the rapid heating device, temperature of the annealing furnace, heated length, and so on. The value of K used in the present example is indicted in Table 1.
Regarding some of the hot-rolled steel strips, the influence FT of the furnace temperature in the heating zone subsequent to the rapid heating device was determined from Expression (2) described above, and the determined influence FT was taken into account in setting the heating temperature on the exit side of the rapid heating device.
The furnace temperature of the annealing furnace 1 (heating zone 3 or the soaking zone 4) was adjusted between 800 to 1100° C.
The heating accuracy of each annealing condition described above was evaluated by splitting each hot-rolled steel strip into 20 sections in the longitudinal direction, and determining the difference between the average steel strip temperature in each split section and the target soaking temperature (1050° C.), and then identifying the maximum value among them. Note that the weight of each hot-rolled steel strip (coil) used in the present example was about 20 tons, and the difference between the preset thickness (preset value of the rolling thickness) and the actually measured thickness of the hot-rolled steel strip was 200 μm at maximum in the front and rear end portions of the hot-rolled steel strip, and was about 50 μm at maximum in the middle portion thereof.
Table 1 also shows the results. Note that in Table 1, the line speed and the LSD at the position of the first ⅓ of each hot-rolled steel strip in the longitudinal direction are respectively indicated by Line Speed 1 and LSD1, those at the position of the second ⅓ of each hot-rolled steel strip in the longitudinal direction are respectively indicated by Line Speed 2 and LSD2, and those at the position of the third ⅓ of each hot-rolled steel strip in the longitudinal direction are respectively indicated by Line Speed 3 and LSD3.
Table 1 shows that Comparative Examples (Test Nos. 18 to 22 and 27) of the conventional technology, in which no rapid heating device was used, exhibited significant variations in the temperatures of the steel strip on the exit side of the heating zone and the exit side of the soaking zone. This is largely due to the influence of the variation in the thickness and threading speed of the steel strip. Meanwhile, even in the cases where a rapid heating device was used (Test Nos. 23 to 26 and 28), the steel strip temperature on the exit side of the heating zone and the exit side of the soaking zone significantly varied under the conditions where the heating temperature of the rapid heating device was set constant without considering the influence of LSDt.
In contrast, in Invention Examples (Test Nos. 1 to 16) in which the heating temperature for the steel strip on the exit side of the rapid heating device was changed in accordance with the variation in LSDt determined from the actually measured thickness of the steel strip, it is observed that the variation in the steel strip temperature on the exit side of the heating zone and the exit side of the soaking zone could be reduced even in the presence of the thickness variation or threading speed variation of the steel strip.
In particular, in Invention Example (Test No. 17), in which the heating temperature for the steel strip on the exit side of the rapid heating device was set by considering the influence FT of the furnace temperature in the rear stage of the heating zone, it is observed that the difference between the steel strip temperature on the exit side of the heating zone and the exit side of the soaking zone and the target heating temperature can be further reduced.
FT can be derived as follows.
(The target annealing temperature for the sheet)−(the sum of the respective sections of the annealing furnace: ΔT1+ΔT2+ΔT3 . . . ΔTn)−(K*(LSDt−LSDs)+250)=FT
The above expression changes with time and allows the steel strip to almost reach the target sheet temperature by finely adjusting FT (the temperature on the exit side of the rapid heating device).
Although the above example was conducted under particular conditions of the thickness, width, and threading speed of the steel strip and the annealing temperature, the present invention is also applicable under different conditions.
The technology of the present invention is directed to minimize the variation in the quality of product sheets by suppressing the variation in the steel strip temperature on the exit side of the heating zone, and thus controlling the annealing temperature (soaking temperature) for the steel strip to a predetermined temperature with high accuracy. The technology of the present invention is applicable to not only a hot-rolled steel strip for an electrical steel sheet but also other metal strips for which the annealing temperature needs to be controlled.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-175709 | Oct 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/038751 | 10/18/2022 | WO |
