Annealing method for hot rolled steel strip

Abstract

Provided is a method for annealing a hot-rolled steel strip, the method controlling the annealing temperature with high accuracy to obtain good magnetic properties throughout the length of the steel strip. In a method for annealing a hot-rolled steel strip for an electrical steel sheet containing 1.6 to 5.0 mass% of Si using a continuous annealing facility in which a heating strip, a soaking strip, and a cooling strip are disposed in order from the upstream side, the heating conditions in the annealing facility are determined from either temperature distribution information of a slab in the length direction thereof during slab heating, or temperature distribution information of the steel strip in the length direction thereof during hot rolling. Further, for the heating conditions in the anneal facility, a rapid heating device is disposed on the upstream side of the soaking strip, a sheet thickness meter is disposed on the upstream side of the rapid heating device, and the steel strip heating temperature of the rapid heating device is set in consideration of the value of LSD, which is defined by the thickness (t) of the steel strip measured by the sheet thickness meter and a sheet passing speed (LS) of the steel strip during annealing, so as to achieve a target annealing temperature. Herein, LSD=t×LS.

Description

[Technical field]

[0001] The present invention relates to a method for annealing a hot-rolled steel strip (hot-rolled steel strip) used in the manufacture of electrical steel sheets. [Background technology]

[0002] It is known that annealing (hot-rolled sheet annealing) of a hot-rolled steel strip for electrical steel sheet containing 1.6 to 5.0 mass% Si before cold rolling improves the magnetic properties of the product sheet, that is, the Goss orientation can be highly developed. The above-mentioned hot-rolled sheet annealing is usually performed using a continuous annealing facility equipped with a heating zone, a soaking zone, and a cooling zone.

[0003] It is known that the annealing temperature in the above-mentioned hot-rolled steel strip annealing has a large effect on the magnetic properties of the product steel strip, and it is necessary to control the steel strip temperature to be constant and uniform over the entire length and width of the hot-rolled steel strip. Therefore, in the above-mentioned continuous annealing equipment, the furnace temperature of the soaking zone is controlled to be constant in order to control the heating temperature of the hot-rolled steel strip to be annealed within a suitable range.

[0004] However, in the above-mentioned annealing method in which only the furnace temperature of the conventional soaking zone is controlled, it is difficult to suppress the variation in the thickness in the longitudinal direction of the steel strip and the variation in the heating temperature of the steel strip due to the variation in the threading speed, and to control it to a suitable range. Specifically, since the hot-rolled steel strip is before cold rolling, has a large thickness, and has a large heat capacity (amount of heat required to raise the temperature of an object by 1°C), it requires a larger amount of heat for heating than the cold-rolled steel strip. In addition, since the hot-rolled steel strip has a large thickness, the thickness variation with respect to the rolled thickness is large, especially at the front and rear ends of the steel strip, so the heat capacity varies greatly even within the same steel strip. Therefore, the hot-rolled steel strip has a large effect on the furnace temperature of the annealing furnace compared to the cold-rolled steel strip, and it is difficult to maintain the furnace temperatures of the heating zone and the soaking zone at a specified temperature and heat the hot-rolled steel strip to the target soaking temperature.

[0005] However, there are few techniques disclosed for controlling the heating temperature in hot-rolled sheet annealing with high accuracy. For example, Patent Document 1 discloses that in a furnace for heating a slab, the temperature is higher in the part (skid part) that contacts the rod-shaped skid that holds the slab and the part (non-skid part) that does not contact the slab. This indicates that the state of precipitates and the solid solution state of inhibitors change, and therefore the optimal heating conditions for obtaining good magnetic properties in annealing after hot rolling change. Patent Document 2 discloses a technique for reducing the aging index of a steel sheet before final cold rolling in the manufacture of grain-oriented electrical steel sheets by controlling the cooling rate in hot-rolled sheet annealing. Patent Document 3 discloses a hot-rolled sheet annealing technique for Si-containing steel sheets in which a steel strip is rapidly heated in hot-rolled sheet annealing. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2006-206997 A [Patent Document 2] JP 2016-000856 A [Patent Document 3] JP 2018-066040 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the technology of the above Patent Document 1 only discloses a slab heating method for eliminating the variation in the magnetic properties of the product steel strip, and does not disclose a method for controlling the temperature of the steel sheet in hot-rolled sheet annealing with high precision. In addition, the technology of the above Patent Document 2 discloses improving the hot-rolled sheet structure of the grain-oriented electrical steel sheet by performing hot-rolled sheet annealing, but does not mention the necessity of controlling the heating temperature of the hot-rolled sheet annealing with high precision or the means for doing so. In addition, the technology of the above Patent Document 3 uses rapid heating in hot-rolled sheet annealing as a means for improving the descaling property after annealing, and does not mention anything about increasing the precision of the heating temperature after rapid heating.

[0008] As described above, in the manufacture of electrical steel sheets, highly accurate control of the heating temperature in the annealing of hot-rolled sheets is extremely important from the viewpoint of improving magnetic properties. In addition, since the temperature history of the slab during slab heating greatly affects the solid solution state of precipitates and inhibitors in the steel, it is necessary to anneal the hot-rolled steel strip after hot rolling at an optimal temperature according to the temperature history of the slab. However, hot-rolled steel strips are characterized by their large thickness, poor thickness accuracy, and large thickness variation, which causes fluctuations in the furnace temperature of the annealing furnace, and therefore there are limitations to controlling the heating temperature of the steel strip with high precision.

[0009] Therefore, in order to solve the above-mentioned problems associated with the conventional technology, the present invention aims to propose an annealing method for hot-rolled steel strip, which enables the annealing temperature of the steel strip to be controlled with high precision to an annealing temperature that is favorable for magnetic properties, thereby making it possible to obtain excellent magnetic properties over the entire length of the steel strip. [Means for solving the problem]

[0010] The inventors have conducted extensive research to solve the above problems. As a result, they have found that in order to perform annealing favorable for magnetic properties on a hot-rolled steel strip for an electrical steel sheet using a continuous annealing facility having an annealing furnace consisting of a heating zone, a soaking zone, and a cooling zone, it is necessary to determine the target annealing temperature (target soaking temperature) in the longitudinal direction of the steel strip in the annealing facility in consideration of the temperature distribution information in the longitudinal direction of the slab during slab heating, or the temperature distribution information in the longitudinal direction of the steel strip during hot rolling. In addition, they have found that in order to heat the steel strip to the target annealing temperature, it is important to arrange a rapid heating device upstream of the soaking zone, arrange a thickness gauge upstream of the rapid heating device, and set the steel strip heating temperature of the rapid heating device in consideration of the LSD value defined by the thickness of the steel strip measured by the thickness gauge and the threading speed of the steel strip during annealing, and have developed the present invention.

[0011] That is, the method for annealing a hot-rolled steel strip for electrical steel sheet according to the present invention is configured as follows. [1] A method for annealing a hot-rolled steel strip for electrical steel sheet obtained by heating and hot-rolling a slab containing 1.6 to 5.0 mass% Si, using continuous annealing equipment having a heating zone, a soaking zone, and a cooling zone arranged in this order from the upstream side, the method determines a target annealing temperature in the longitudinal direction of the steel strip in the annealing equipment based on information on the temperature distribution in the longitudinal direction of the slab during the slab heating or information on the temperature distribution in the longitudinal direction of the steel strip during the hot rolling, and sets heating conditions for the steel strip in the annealing equipment. [2] In the above [1], the heating conditions of the steel strip in the annealing equipment include arranging a rapid heating device upstream of the soaking zone and a thickness gauge upstream of the rapid heating device, and setting the steel strip heating temperature of the rapid heating device so as to achieve the target annealing temperature based on the LSD value defined by the following formula (1) based on the thickness of the steel strip measured by the thickness gauge and the threading speed of the steel strip during annealing. Note LSD = t × LS (1) Here, t is the thickness of the steel strip (mm) and LS is the threading speed of the steel strip (m / min). [3] In the above [1] or [2], a method for annealing a hot-rolled steel strip, comprising determining a target annealing temperature in the longitudinal direction of the steel strip in the annealing equipment using skid position information during heating of the slab. [4] In the above [3], a method for annealing a hot-rolled steel strip is provided, in which a target annealing temperature in the longitudinal direction of the steel strip during the annealing is set using the skid position information during the slab heating as well as the time the slab was in the furnace during the slab heating. [5] In any one of the above [2] to [4], there is provided a method for annealing a hot-rolled steel strip, in which the steel strip heating temperature of the rapid heating device is set by feeding back information on a furnace temperature downstream of the rapid heating device. Effect of the Invention

[0012] According to the present invention, it is possible to control with high precision the steel strip temperature on the exit side of the heating zone, and therefore the steel strip temperature on the exit side of the soaking zone, to a target annealing temperature that is favorable for magnetic properties, which greatly contributes to improving the magnetic properties of the product sheet and reducing the occurrence of defects. Here, in the present invention, the heating conditions of the annealing equipment are set so as to achieve a target annealing temperature (target soaking temperature) in the longitudinal direction of the steel strip, which is determined taking into consideration the temperature distribution in the longitudinal direction of the slab during heating of the slab before hot rolling, or the temperature distribution in the longitudinal direction of the hot-rolled steel strip during hot rolling. Specifically, a rapid heating device is arranged upstream of the soaking zone, and a thickness gauge is arranged upstream of the rapid heating device, and the heating temperature of the steel strip by the rapid heating device is set so as to achieve the target annealing temperature, taking into consideration the LSD value defined by the thickness of the steel strip measured by the thickness gauge and the threading speed of the steel strip during annealing. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing an example of a continuous annealing facility used for annealing a hot-rolled steel strip according to the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing another example of continuous annealing equipment used for annealing the hot-rolled steel strip of the present invention. [Diagram 3]FIG. 1 is a diagram for explaining the relationship between the slab heating temperature distribution and the target annealing temperature of a hot-rolled steel strip, where (a) shows the temperature distribution in the longitudinal direction of the slab during slab heating, and (b) shows the target annealing temperature in the longitudinal direction of the hot-rolled steel strip determined taking into account the temperature distribution in (a) above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In one embodiment of the present invention, when annealing a hot-rolled steel strip for an electrical steel sheet (hot-rolled sheet annealing) using an annealing facility having a heating zone, a soaking zone, and a cooling zone arranged in this order, the heating conditions of the annealing facility are set so as to achieve a target annealing temperature in the longitudinal direction of the steel strip that is favorable for magnetic properties, which is determined from the temperature distribution in the longitudinal direction of the slab during slab heating or the temperature distribution in the longitudinal direction of the steel strip during hot rolling. In another embodiment, a rapid heating device is arranged upstream of the soaking zone of the annealing facility, and a thickness gauge is arranged upstream of the rapid heating device, and the heating temperature of the steel strip by the rapid heating device is set so as to achieve the target annealing temperature, taking into consideration the LSD value defined by the thickness of the steel strip measured by the thickness gauge and the threading speed of the steel strip during annealing.

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 1 is a schematic diagram showing the first half of a continuous annealing facility used for annealing a hot-rolled steel strip for an electrical steel sheet (hot-rolled sheet annealing) according to the present invention, which is provided with a heating zone, a soaking zone, and a cooling zone from the upstream side. Usually, the hot-rolled sheet annealing is performed by heating the steel strip S in the heating zone 3 so that the steel strip temperature at the exit side of the heating zone reaches a target soaking temperature, then holding the steel strip S at the soaking temperature in the soaking zone 4 for a predetermined time, and then cooling the steel strip S in a cooling zone (not shown). In this case, the furnace temperatures of the heating zone 3 and the soaking zone 4 and the passing speed of the steel strip S are kept constant from the viewpoint of holding the steel strip at the predetermined soaking temperature for a predetermined time, as long as there is no change in the thickness or passing speed of the steel strip.

[0016] <Determining the target annealing temperature> In the hot rolling process, the slab is heated to concentrate the crystal grains in the Goss orientation in the secondary recrystallization, and the solid solution of precipitates called inhibitors is controlled. However, even if the slab is uniformly heated in the heating furnace when it is removed from the heating furnace, the part of the slab that is in contact with the water-cooled slab-holding rod called a skid installed in the heating furnace is cooled. Therefore, the temperature distribution of the slab when it is removed varies greatly in the length direction depending on the position of the skid and the time it is in the furnace. In addition, the temperature history of the slab is different between the part of the slab that is in contact with the skid part (skid part) and the part that is not in contact with the skid part (non-skid part). Therefore, the state of precipitates in the steel strip changes in the length direction during the process after hot rolling, which has a large effect on the recrystallization temperature, etc., and the magnetic properties of the final product vary even if it is manufactured under the same conditions.

[0017] Therefore, in the present invention, the influence of temperature variation in the longitudinal direction of the slab on the state of precipitates in the hot-rolled steel strip, the recrystallization temperature, and ultimately the magnetic properties of the final product is reduced. To this end, as shown in Fig. 3, the target annealing temperature in the longitudinal direction of the steel strip in the annealing (hot-rolled sheet annealing) performed on the steel strip after hot rolling is changed according to the longitudinal temperature distribution of the slab during slab heating, thereby controlling the state of precipitates and the recrystallization temperature to be uniform in the longitudinal direction of the steel strip.

[0018] The target annealing temperature in the longitudinal direction of the steel strip in the annealing equipment, which is determined based on the temperature distribution in the longitudinal direction of the slab, is desirably set after verifying the effect on the magnetic properties of the final product, since the magnitude of the effect varies depending on the steel type and the type and amount of inhibitor. Here, the target annealing temperature is preferably set by using a method for processing large amounts of data, such as statistical processing, machine learning, or AI, based on the verification.

[0019] The target annealing temperature for annealing after hot rolling is set as follows: In the portion of the slab where the heating temperature is low, the solid solution of the inhibitor is likely to be insufficient, and during annealing after hot rolling, crystal grains with an orientation unfavorable for magnetic properties may grow. On the other hand, in the portion of the slab where the heating temperature is high, the crystal grains become coarse. Therefore, a lot of energy is required to grow crystals favorable for magnetic properties, so the target annealing temperature is set high.

[0020] Furthermore, when intermediate annealing is performed in the cold rolling process, the effect of the slab heating temperature on the magnetic properties is mitigated to some extent. However, when intermediate annealing is not performed, it is more preferable to set the annealing temperature after hot rolling that reflects the temperature distribution in the longitudinal direction of the slab.

[0021] Furthermore, when it is difficult to actually measure the temperature distribution during slab heating, it may be obtained by numerical analysis from information such as the skid position, furnace time, furnace temperature, etc. This is because if the furnace time and skid position change, the temperature distribution of the slab during heating changes significantly in the thickness and longitudinal directions. Furthermore, when the slab is heated by induction heating, it is preferable to perform a numerical analysis taking into account information on the arrangement of the induction coil and the power output.

[0022] Also, instead of the temperature distribution in the longitudinal direction of the slab, it is possible to use the temperature distribution in the longitudinal direction of the steel strip during rough rolling in the hot rolling process which is the process immediately following the rough rolling. This is because the surface temperature distribution in the longitudinal direction of the steel strip has the same tendency as the temperature distribution in the longitudinal direction of the slab during slab heating. Since the steel strip after rough rolling is thinner than the slab, the surface temperature of the steel strip after rough rolling can be directly measured by a surface thermometer such as a radiation thermometer. In this case, in order to improve the measurement accuracy, it is preferable to measure the surface temperature of the steel strip immediately after descaling, which removes the oxide film formed on the steel strip surface.

[0023] <Setting the heating temperature of steel strips using a rapid heating device> As described above, after the target annealing temperature in the longitudinal direction of the steel strip in the annealing equipment is determined, the heating conditions of the steel strip in the annealing equipment, for example, the heating temperatures in the heating zone and soaking zone, are set. 1 is a diagram showing the first half of a continuous annealing facility having an annealing furnace 1 used for annealing a hot-rolled steel strip, in which a heating zone 3, a soaking zone 4, and a cooling zone are arranged from the upstream side, and shows an example in which a rapid heating device 2 is arranged upstream of the heating zone 3 as one embodiment of the present invention. The rapid heating device 2 has a function of heating the steel strip to a target annealing temperature determined from the temperature distribution in the longitudinal direction of the slab during slab heating, or the temperature distribution in the longitudinal direction of the steel strip during hot rolling.

[0024] However, there is a certain amount of thickness variation in the length direction of the steel strip, and especially a large thickness variation exists at the leading and trailing ends of the steel strip. In addition, the threading speed may be changed suddenly during the annealing of the steel strip. When such a large thickness variation or a sudden change in the threading speed occurs, the steel strip cannot be heated to the target temperature by simply controlling the furnace temperature of the heating zone or the soaking zone to be constant, so the set temperature of the furnace temperature must be changed. However, a sudden change in the set temperature of the furnace temperature causes overshooting and hunting of not only the furnace temperature but also the steel strip temperature. In addition, it takes time for the furnace temperature to change, and an instantaneous change in the furnace temperature cannot be expected. Therefore, it is desirable to keep the furnace temperature of the annealing furnace 1 as constant as possible or to change it gradually.

[0025] Furthermore, as described above, a hot-rolled steel strip is thicker and has a larger heat capacity than a cold-rolled steel strip, so that it is difficult to follow the change in furnace temperature of the heating zone and also affects the furnace temperature of heating zone 3, making it difficult to heat the steel strip to a target temperature. Therefore, in heating zone 3, it is desired to heat the steel strip so that the temperature of the steel strip on the exit side of the heating zone always becomes a predetermined target temperature regardless of the fluctuation in the thickness of the steel strip and the threading speed.

[0026] The ultimate objective of hot-rolled steel strip annealing is to perform heat treatment in the soaking zone 4 by holding the steel strip S at a target annealing temperature (soaking temperature) for a predetermined time. However, in the case of a hot-rolled steel strip, which is thicker than a cold-rolled steel strip, if the steel strip temperature on the exit side of the heating zone differs, the heat amount of the steel strip also changes significantly, so the amount of heat brought into the soaking zone 4 changes, and ultimately the furnace temperature also changes, making it difficult to heat the hot-rolled steel strip to the target soaking temperature. In this sense, too, it is preferable to set the heating temperature of the rapid heating device 2 so that the steel strip temperature on the exit side of the heating zone 3 is constant.

[0027] Therefore, when hot-rolled steel strips for electrical steel sheets are annealed using the continuous annealing equipment shown in Fig. 1, a thickness gauge 5 is placed upstream of the heating zone 3, and the LSD defined by the following formula (1) is used based on the thickness of the steel strip measured by the thickness gauge 5 and the passing speed of the steel strip during annealing. That is, the heating temperature of the steel strip S by the rapid heating device 2 is changed according to the value of LSD, so that the steel strip temperature at the exit side of the heating zone is heated to a predetermined target temperature. The LSD is an index proportional to the heat capacity of the steel strip. LSD = t × LS (1) Here, t is the thickness of the steel strip (mm) and LS is the threading speed of the steel strip (m / min).

[0028] Specifically, in this embodiment, the larger the LSD value of the hot rolled steel strip, the higher the heating temperature of the steel strip S by the rapid heating device 2 is set, and conversely, the smaller the LSD value of the hot rolled steel strip, the lower the heating temperature by the rapid heating device 2 is set. More specifically, the thicker the steel strip thickness is, including minute thickness variations of the steel strip, the higher the steel strip temperature at the exit side of the rapid heating device, i.e., the heating temperature by the rapid heating device 2 is set than the conventional heating temperature that does not consider LSD. Conversely, the thinner the strip thickness is, the lower the steel strip temperature at the exit side of the rapid heating device, i.e., the heating temperature by the rapid heating device 2 is set than the conventional heating temperature that does not consider LSD. Similarly, when the threading speed increases, the steel strip temperature at the exit side of the rapid heating device, i.e., the heating temperature by the rapid heating device 2 is set higher than the conventional heating temperature that does not consider LSD. Conversely, when the strip threading speed decreases, the steel strip temperature at the exit side of the rapid heating device 2, i.e., the heating temperature by the rapid heating device 2, is set lower than the conventional heating temperature that does not take LSD into consideration.

[0029] In this way, by changing the heating temperature of the steel strip S in the rapid heating device 2 while taking the LSD value into consideration, even if the thickness t or the threading speed LS of the hot rolled steel strip changes suddenly, it becomes possible to suppress fluctuations in the steel strip temperature at the outlet side of the heating zone without changing the furnace temperature setting of the heating zone 3. Furthermore, it becomes possible to suppress the furnace temperature fluctuations in the soaking zone 4 and control the temperature of the hot rolled steel strip to the target soaking temperature with high accuracy.

[0030] Here, the rapid heating device 2 may be of any type as long as it can change the steel strip temperature with good responsiveness. In addition, any method of heating the steel strip S may be suitably used as long as it is induction heating (solenoid type or transverse type), electric current heating, near-infrared heating, or the like, which have a long track record.

[0031] The rapid heating device 2 is installed upstream of the soaking zone 4, since the purpose of this device is to control with high precision the steel strip temperature at the exit of the heating zone, and therefore the steel strip temperature at the exit of the soaking furnace. In this case, it may be installed upstream of the heating zone, as shown in Fig. 1, or it may be installed inside the heating zone 3 (including the case where it is present between the divided heating zones 3), as shown in Fig. 2. However, when an induction heating device, particularly a solenoid type device, is used as the rapid heating device 2, it is difficult to heat to a temperature higher than the Curie point, so it is preferable to install it in the front or middle stage when the heating zone 3 is divided into three stages from the upstream side, i.e., in the front stage, middle stage, and rear stage, and not in the rear stage.

[0032] Furthermore, the greater the heating capacity of the rapid heating device 2, the more preferable it is in order to accommodate large variations in the thickness of the hot-rolled steel strip and the threading speed. As a guideline, although it depends on the temperature range (position) where the rapid heating device is installed and the "temperature-specific heat" relationship of the steel strip, if it is desired to change the heating temperature of a steel strip with a thickness of about 2 mm at the exit side of the heating zone by about ±20°C, a heating capacity (amount of temperature rise) of about 50 to 100°C will be sufficient to accommodate variations in the thickness and threading speed.

[0033] As explained above, the setting of the heating temperature of the steel strip S by the rapid heating device 2 can be changed according to the fluctuation of the LSD value, i.e., (strip thickness x strip passing speed), so as to achieve the set target annealing temperature (soaking temperature), which can sufficiently contribute to the high accuracy of the heating temperature in the longitudinal direction of the steel strip S. However, in order to control the steel strip temperature at the exit side of the heating zone 3 with even higher accuracy, it is preferable to consider the width of the hot rolled steel strip in addition to the above LSD value. This is because the heat capacity of the steel strip changes in proportion to the width of the steel strip, so that the amount of heat required to heat the steel strip and the amount of heat removed from the furnace body also change, which in turn affects the steel strip temperature at the exit side of the heating zone and the furnace temperature of the heating zone and the soaking zone. In particular, when using an annealing facility having a heating zone or soaking zone with a small heat capacity and a furnace temperature that is easily changed, it is preferable to set the heating temperature of the rapid heating device taking into account the width of the steel strip.

[0034] Furthermore, in order to control the steel strip temperature on the exit side of the heating zone with higher accuracy, it is preferable to take into account furnace temperature information of the heating zone 3 after the rapid heating device when setting the heating temperature of the rapid heating device 2. For example, if the heating capacity of the rapid heating device 2 is small, and a large change in the heat capacity of the steel strip occurs due to a change in the thickness or threading speed of the hot rolled steel strip or a change in the LSD, the rapid heating device 2 alone may not be able to adequately respond, so it is possible to consider changing the furnace temperature setting of the heating zone 3. In such a case, by feeding back constantly changing furnace temperature information after the rapid heating device 2 and reflecting it in the setting of the heating temperature of the rapid heating device 2, it becomes possible to control the steel strip temperature on the exit side of the heating zone 3 with higher accuracy.

[0035] When the furnace temperature information is fed back, the increase in the steel strip temperature at the exit side of the heating zone can be calculated using the following formula (2).

[0036]

number

[0037] In order to control the steel strip temperature at the exit of the heating zone with even greater accuracy, it is preferable to utilize information on the steel strip temperature at the exit of the heating zone, the furnace temperature in the soaking zone, and the steel strip temperature at the exit of the soaking zone in addition to the above-mentioned LSD, steel strip width, and furnace temperature of the heating zone after the rapid heating device, in determining the steel strip heating temperature in the rapid heating device 2.

[0038] Here, the hot-rolled steel strip for electrical steel sheet to which the present invention is directed preferably contains Si in the range of 1.6 to 5.0 mass%. Since Si is an effective element for increasing the resistivity of steel and reducing iron loss, it is preferable to contain 1.6 mass% or more. However, if the Si content exceeds 5.0 mass%, not only does the magnetic flux density decrease, but the steel becomes embrittled, cracks are generated during cold rolling, and the manufacturability is greatly reduced. The preferred range is 2.0 to 3.8 mass%.

[0039] In addition to Si, the hot-rolled steel strip may further contain known inhibitor-forming components for inducing secondary recrystallization, or may contain known additive elements for improving magnetic properties and mechanical properties.

[0040] According to the hot-rolled sheet annealing of the present invention described above, heat treatment can be performed over the entire length of the hot-rolled steel strip in the longitudinal direction (rolling direction) at an appropriate temperature that corresponds to the longitudinal temperature distribution during slab heating, as well as the longitudinal thickness variation and the threading speed variation of the steel strip. Therefore, it is possible to obtain a product having excellent magnetic properties without secondary recrystallization defects or crystal orientation defects over the entire length of the steel strip. EXAMPLES

[0041] A steel slab (slab weight: 20 tons) for electrical steel sheet containing 3.5 mass% Si was produced by continuous casting, heated to a temperature of 1200°C or higher, and then hot-rolled to a hot-rolled steel strip with a thickness of 2 mm. Next, using a continuous annealing facility shown in Figure 1, in which a thickness gauge 5 and a rapid heating device 2 are placed upstream of the heating zone, hot-rolled sheet annealing was performed with a target annealing temperature (soaking temperature) and soaking time of 1050°C for 30 seconds, and then the sheet was pickled and descaled. In the above-mentioned hot-rolled sheet annealing, the target annealing temperature (target soaking temperature) was gradually changed in the length direction of the steel strip in consideration of the temperature distribution in the length direction of the slab during slab heating, or the temperature distribution in the length direction of the steel strip during rough rolling in hot rolling, or the skid position information during slab heating, as shown in Table 1. Also, an induction heating device was used as the rapid heating device in the above-mentioned annealing equipment, and as shown in Table 1, the heating temperature by the device was gradually changed in the length direction of the steel strip in consideration of the value of LSD (sheet thickness x line speed) so that the above-mentioned target annealing temperature was obtained for some steel strips. During this process, the furnace temperatures in the heating zone and soaking zone were kept constant, and the surface temperature of the steel strip was measured with a radiation thermometer at the exit side of the soaking zone, and the difference ΔT from the target annealing temperature determined in consideration of the temperature distribution in the longitudinal direction of the steel strip was successively calculated, with the maximum value ΔTmax being shown in Table 1. The target annealing temperature was set using formula (2) as the temperature at which the magnetic properties were most improved based on past records of hot-rolled sheet annealing temperatures and magnetic properties.

[0042] Next, the above pickled steel strip was cold rolled to an intermediate thickness of 1.5 mm, and then the steel strip was divided into two longitudinally. One was subjected to intermediate annealing at 1100°C for 20 seconds, and the other was subjected to a second cold rolling without intermediate annealing to produce a cold-rolled steel strip with a final thickness of 0.23 mm. Next, the steel strip after the second cold rolling was subjected to decarburization annealing, which also served as primary recrystallization annealing, at a temperature of 840°C for 100 seconds, after which an annealing separator mainly made of MgO was applied to the surface of the steel sheet and dried, after which it was subjected to finish annealing consisting of secondary recrystallization annealing and purification treatment at a temperature of 1200°C for 10 hours. The atmospheric gas for the finish annealing was H 2, and N during other heating (including secondary recrystallization annealing) and cooling. 2 It was decided.

[0043] Next, the steel strip after the final annealing obtained as described above was divided into 10 sections in the length direction, and test pieces for measuring magnetic properties were taken from each section. 8 Measure in accordance with the Japanese Industrial Standard JISC2553, and measure the difference between the maximum and minimum values, ΔB 8 The results are shown in Table 1.

[0044] From the above results, it can be seen that the steel strips Nos. 1 to 6 and 9 to 12 (invention examples) which were subjected to hot-rolled sheet annealing under conditions compatible with the present invention were able to control the difference between the target annealing temperature, taking into account the effect of the slab heating temperature on the magnetic properties, and the steel strip temperature at the exit of the soaking zone to a small value, thereby producing grain-oriented electrical steel sheets with small variation in magnetic properties within the same steel strip. In contrast, the target annealing temperature for hot strip annealing was determined taking into account the skid position information during slab heating. However, in the case of steel strips No. 13 and 14 (comparative examples) in which the LSD value was not taken into account in setting the heating temperature of the rapid heating device, the difference ΔT in the steel strip temperature at the outlet of the soaking zone from the target annealing temperature of the annealing equipment was large, so the variation in magnetic properties in the longitudinal direction within the steel strip ΔB 8 has become bigger. In addition, even if the heating temperature of the rapid heating device is set taking into account the LSD value, the steel strips No. 15 and 16 (comparative examples) in which the target annealing temperature of the hot-rolled sheet annealing was not set taking into account the temperature distribution in the longitudinal direction of the slab during slab heating, or the temperature distribution in the longitudinal direction of the steel strip after rough rolling in hot rolling, or the skid position information during slab heating, could not eliminate the influence of temperature unevenness during slab heating. Therefore, although the variation in the steel strip temperature relative to the target annealing temperature could be reduced, the variation in the magnetic properties became large. In addition, in the steel strips Nos. 7 and 8 (conventional examples) in which the target annealing temperature for hot-rolled strip annealing was not set taking into consideration the temperature distribution in the longitudinal direction of the slab during slab heating, or the temperature distribution in the longitudinal direction of the steel strip after rough rolling in hot rolling, or the skid position information during slab heating, and the LSD value was not taken into consideration when setting the heating temperature of the rapid heating device, the difference ΔT in the steel strip temperature at the outlet of the soaking zone from the target annealing temperature was large, and the effect of temperature unevenness during slab heating on the magnetic properties could not be eliminated. Therefore, the variation ΔB in the magnetic properties in the longitudinal direction of the steel strip 8 became the largest. In addition, under any of the conditions in the above examples, by performing intermediate annealing in the cold rolling process, the influence of the slab heating temperature distribution was reduced, and the variation in magnetic properties in the longitudinal direction of the steel strip was reduced.

[0045] [Table 1] [Industrial Applicability]

[0046] The technology of the present invention can be applied not only to hot-rolled steel strips for electrical steel sheets, but also to all metal strips that need to be annealed while taking into account the longitudinal variation of the slab heating temperature. [Explanation of symbols]

[0047] S: Steel strip 1: Annealing furnace 2: Rapid heating device 3: Heating zone 4:Solid temperature 5: Plate thickness gauge

Claims

1. A method for annealing a hot-rolled steel strip for an electrical steel sheet obtained by heating and hot-rolling a slab containing 1.6 to 5.0 mass% Si using a continuous annealing facility having a heating zone, a soaking zone, and a cooling zone arranged in this order from the upstream side, comprising: A target annealing temperature in the longitudinal direction of the steel strip in the annealing facility is determined based on temperature distribution information in the longitudinal direction of the slab during the slab heating or temperature distribution information in the longitudinal direction of the steel strip during hot rolling, and heating conditions for the steel strip in the annealing facility are set; Here, the method for annealing a hot-rolled steel strip is characterized in that the target annealing temperature is a target steel strip temperature.

2. The heating conditions of the steel strip in the annealing equipment are as follows: A rapid heating device is arranged upstream of the soaking zone, and a thickness gauge is arranged upstream of the rapid heating device; The method for annealing a hot-rolled steel strip according to claim 1, characterized in that the heating temperature of the steel strip in the rapid heating device is set so as to achieve the target annealing temperature from the LSD value defined by the following formula (1) based on the thickness of the steel strip measured by the thickness gauge and the threading speed of the steel strip during annealing. Note LSD=t×LS...(1) Here, t is the thickness of the steel strip (mm), and LS is the threading speed of the steel strip (m / min).

3. 3. The method for annealing a hot-rolled steel strip according to claim 2, wherein a target annealing temperature in the longitudinal direction of the steel strip in the annealing facility is determined using skid position information during heating of the slab.

4. The method for annealing a hot-rolled steel strip according to claim 3, characterized in that in addition to the skid position information during the slab heating, the target annealing temperature in the longitudinal direction of the steel strip during the annealing is set using the time the slab was in the furnace during the heating.

5. 5. A method for annealing a hot-rolled steel strip according to claim 2, wherein the heating temperature of the steel strip in the rapid heating device is set by feeding back information on a furnace temperature downstream of the rapid heating device.

6. 2. The method for annealing a hot-rolled steel strip according to claim 1, wherein a target annealing temperature in the longitudinal direction of the steel strip in the annealing facility is determined using skid position information during heating of the slab.

7. The method for annealing a hot-rolled steel strip according to claim 6, characterized in that in addition to the skid position information during the slab heating, the target annealing temperature in the longitudinal direction of the steel strip during the annealing is set using the time the slab was in the furnace during the heating.

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