Method for producing oriented electromagnetic steel sheet

Abstract

Provided is a method for producing an oriented electromagnetic steel sheet capable of improving magnetic flux density. When a steel material having a predetermined component composition is subjected to hot rolling, cold rolling, decarburization annealing, application of an annealing separation agent, and finish annealing in sequence, the present invention controls: a storage condition between final cold rolling of a cold rolled plate coil and decarburization annealing; and a temperature fluctuation management condition provided in the middle of the temperature raising process of the decarburization annealing.

Description

Manufacturing method of grain-oriented electrical steel sheet

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet having a high magnetic flux density.

[0002] Grain-oriented electrical steel sheet is a soft magnetic material primarily used as a transformer core material, and has a crystalline structure in which the <001> orientation, the axis of easy magnetization of iron, is highly aligned in the rolling direction of the steel sheet. In grain-oriented electrical steel sheet, the finer the grains, the lower the iron loss.

[0003] Here, when rapidly heating a steel sheet (cold-rolled sheet) in the decarburization annealing step in the manufacturing process of grain-oriented electrical steel sheet, the temperature is adjusted for a predetermined time in a temperature range where recovery occurs, thereby making the temperature inside the steel sheet uniform and achieving the effects of rapid heating across the entire width of the steel sheet.

[0004] It is also known that by adjusting the temperature for a predetermined time in a temperature range where such recovery occurs, the <111> / / ND orientation is preferentially recovered, reducing the <111> / / ND orientation after primary recrystallization, and instead increasing Goss nuclei, resulting in finer grains after secondary recrystallization.

[0005] These effects make it possible to obtain a grain-oriented electrical steel sheet with low iron loss (see, for example, Patent Document 1).

[0006] International Publication No. 2014 / 017589

[0007] However, even when the grain-oriented electrical steel sheet manufacturing process described in Patent Document 1 is used, during secondary recrystallization, both ideal Goss grains with high orientation, where the deviation from (110)

[001] is suppressed to 5° or less, and Goss grains with low orientation, where the deviation is more than 5° and 10° or less, grow to large sizes.

[0008] Therefore, one of the challenges to increasing the magnetic flux density remains the need to selectively grow only the ideal Goss grains with high orientation, out of the ideal Goss grains with high orientation and the misaligned Goss grains with low orientation.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a method for manufacturing a grain-oriented electrical steel sheet that allows highly oriented Goss grains to grow preferentially and that can exhibit high magnetic flux density.

[0010] The present inventors have conducted extensive research to achieve the above-mentioned object. As a result, they have focused on performing an aging treatment under specific conditions on a coiled steel sheet (cold-rolled sheet) during the period from the end of final cold rolling to the start of decarburization annealing, which also serves as primary recrystallization annealing, and on temperature management under specific conditions during the temperature rise process of the subsequent decarburization annealing. They have found that controlling these processes can improve the orientation of Goss grains and increase the magnetic flux density of the resulting grain-oriented electrical steel sheet, and have completed the present invention.

[0011] The present invention has been completed based on the above findings. That is, the gist of the present invention is as follows: [1] A method for producing a grain-oriented electrical steel sheet, comprising: hot rolling a steel material containing, by mass%, 0.002 to 0.100% C, 2.00 to 4.50% Si, and 0.01 to 0.50% Mn, with the balance being Fe and unavoidable impurities, to form a hot-rolled sheet; subjecting the hot-rolled sheet to hot-rolled annealing to form a hot-rolled annealed sheet, or subjecting the hot-rolled sheet or the hot-rolled annealed sheet to cold-rolling once or at least twice with intermediate annealing therebetween to form a cold-rolled sheet having a final sheet thickness, subjecting the cold-rolled sheet to decarburization annealing which also serves as primary recrystallization annealing to form a decarburization-annealed sheet, and then applying an annealing separator to the surface of the decarburization-annealed sheet and finish-annealing the decarburization-annealed sheet to obtain a grain-oriented electrical steel sheet, a method for producing a grain-oriented electrical steel sheet, wherein, during the period from the end of final cold rolling in the cold rolling to the start of decarburization annealing, a coil obtained by winding the cold-rolled sheet is stored at an average storage temperature of 150°C or less for a storage time of 3 to 360 hours; and during the temperature-raising process of the decarburization annealing, temperature fluctuation is controlled in a range of -20 to +20°C / s within a range of an average temperature T (°C) and a time t (s) that satisfy the following formulas (1) to (3):

[0012] [2] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein the steel material further contains, in mass%, one or more elements selected from Al: 0.005 to 0.050%, N: 0.0030 to 0.0200%, Se: 0.003 to 0.030%, and S: 0.002 to 0.030%.

[0013] [3] The method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], wherein the steel material further contains one or more selected from at least one of the following groups A to D: Group A: In mass%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.010 to 0.500%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.500%, Sb: 0.005 to 0.500%, and Mo: 0.005 to 0.500%. Group B: In mass% or mass ppm, B: 0.1 to 25.0 ppm, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and Co: 0.002 to 0.050%. Group C: Group D: By mass %, As: 0.0010 to 0.0200%, Pb: 0.0001 to 0.0100%, W: 0.0010 to 0.0100%, and Zn: 0.001 to 0.020%. Group D: By mass %, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ca: 0.001 to 0.020%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Nd: 0.001 to 0.020%, and La: 0.001 to 0.020%.

[0014] According to the present invention, it is possible to provide a method for producing a grain-oriented electrical steel sheet that improves the orientation of Goss grains in the steel sheet (decarburization-annealed sheet) at the completion of primary recrystallization, thereby improving the magnetic flux density of the grain-oriented electrical steel sheet as a finished sheet.

[0015] Magnetic flux density B for each storage time between the end of final cold rolling and the start of decarburization annealing ８ 1 is a graph showing the magnetic flux density B for each average storage temperature from the end of final cold rolling to the start of decarburization annealing. ８ 10 is a graph showing the magnetic flux density B for each temperature rise / fall rate in temperature fluctuation management. ８1 is a graph showing the magnetic flux density B versus the average temperature T and time t in temperature fluctuation management. ８ 10 is a graph summarizing the evaluation results.

[0016] First, we will describe the experiments that inspired the development of the present invention. In this specification, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively. Furthermore, if a unit is attached to only one of the numerical values ​​before and after "to", the same unit is also attached to the other, unless otherwise specified. <Experiment 1> A steel slab containing, by mass%, 0.070% C, 3.35% Si, 0.10% Mn, 0.013% Se, and 0.004% S, with the balance being Fe and unavoidable impurities, was produced by a continuous casting method. This steel slab was heated to a temperature of 1420°C and then hot-rolled to obtain a hot-rolled sheet with a thickness of 2.4 mm. This hot-rolled sheet was then hot-rolled at 1000°C for 50 seconds to obtain a hot-rolled annealed sheet. This hot-rolled annealed sheet was subjected to primary cold rolling to reduce the intermediate thickness to 1.8 mm, intermediate annealing at 1100°C for 20 seconds, and then secondary cold rolling to produce a cold-rolled sheet with a final thickness of 0.27 mm. The finished cold-rolled sheet was wound into a coil.

[0017] Here, after the completion of the secondary cold rolling, which is the final cold rolling, the cold-rolled sheet coils were stored at an average storage temperature of 100°C for different storage times of 1 to 456 hours. More specifically, the storage time was changed in ascending order from 1 hour, 2 hours, 3 hours, and 12 hours, and various conditions were set up up to 456 hours. After each storage, the coils were subjected to H ２ and N ２ In a mixed atmosphere, decarburization annealing, which also served as primary recrystallization annealing, was performed under the conditions of a holding temperature of 840°C and a holding time of 100 seconds. The temperature increase in this decarburization annealing began at a rate of 200°C / s, and during this temperature increase, temperature fluctuation control was performed under the conditions of a temperature increase rate of 10°C / s, an average temperature of 300°C, and a time of 1.0 s. Thereafter, the temperature was again increased to 600°C at a rate of 200°C / s. The temperature was further increased to the above-mentioned holding temperature at a rate of 25°C / s, and after holding for the above-mentioned holding time, the sample was allowed to cool in the atmosphere.

[0018] Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet (decarburized annealed sheet) and dried. Thereafter, finish annealing was performed at a maximum temperature of 1200°C to obtain grain-oriented electrical steel sheets. During this finish annealing, a purification treatment was also performed by holding the steel sheet at 1150°C or higher in a hydrogen atmosphere for 10 hours. The magnetic flux density B ８ The magnetic flux density (when a magnetic field of 800 A / m was applied) was measured, and the results are shown in FIG.

[0019] From FIG. 1, it was found that a grain-oriented electrical steel sheet with high magnetic flux density can be obtained when the storage time (aging time) of the cold-rolled sheet coil from the end of final cold rolling to the start of decarburization annealing satisfies 3 hours or more and 360 hours or less.

[0020] <Experiment 2> A coil of cold-rolled sheet having a final thickness obtained under the same conditions as in Experiment 1 was used. After the completion of the secondary cold rolling, which is the final cold rolling, this coil was stored at different average storage temperatures in the range of 0 to 180°C for a storage time of 200 hours. After each storage period, the coil was subjected to H ２ and N ２ In a mixed atmosphere, decarburization annealing, which also served as primary recrystallization annealing, was performed under the conditions of a holding temperature of 840°C and a holding time of 100 seconds. The temperature increase in this decarburization annealing started at a rate of 200°C / s, and during this temperature increase, temperature fluctuation control was performed under the conditions of a temperature increase rate of 10°C / s, an average temperature of 300°C, and a time of 1.0 s. Thereafter, the temperature was again increased to 600°C at a rate of 200°C / s. The temperature was further increased to the above-mentioned holding temperature at a rate of 25°C / s, and after holding for the above-mentioned holding time, the sample was allowed to cool in the atmosphere.

[0021] Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet (decarburized annealed sheet) and dried. After that, finish annealing was performed at a maximum temperature of 1200°C to obtain a grain-oriented electrical steel sheet. During this finish annealing, a purification treatment was also performed by holding the steel sheet at 1150°C or higher in a hydrogen atmosphere for 10 hours.

[0022] For each grain-oriented electrical steel sheet thus obtained after the final annealing, the magnetic flux density B８ The magnetic flux density (when a magnetic field of 800 A / m was applied) was measured, and the results are shown in FIG.

[0023] From FIG. 2 , it was found that a grain-oriented electrical steel sheet with a high magnetic flux density can be obtained by setting the average storage temperature of the cold-rolled sheet coil in the range of 150°C or less (0°C or more and 150°C or less in the example of FIG. 2 ) from the end of final cold rolling to the start of decarburization annealing.

[0024] <Experiment 3> A coil of cold-rolled sheet having a final thickness obtained under the same conditions as in Experiment 1 was used. This coil was stored at an average storage temperature of 30°C for a storage time of 200 hours after the end of the secondary cold rolling, which is the final cold rolling. ２ and N ２ In a mixed atmosphere, decarburization annealing, which also served as primary recrystallization annealing, was performed under the conditions of a holding temperature of 840°C and a holding time of 100 seconds. The temperature increase during this decarburization annealing began at a rate of 200°C / s, and during this temperature increase, temperature fluctuation control was performed under the conditions of an average temperature of 300°C and a time of 1.0 seconds, with a temperature increase or decrease rate in the range of -30 to +30°C / s. Thereafter, the temperature was again increased to 600°C at a rate of 200°C / s. The temperature was then further increased to the above-mentioned holding temperature at a rate of 25°C / s, and after holding for the above-mentioned holding time, the sample was allowed to cool in the atmosphere.

[0025] Here, when the temperature rise / fall rate in the temperature fluctuation management is marked with a positive sign (+), it is treated as if no sign was attached, meaning that the coil temperature rises over time during the temperature fluctuation management. On the other hand, when the temperature rise / fall rate in the temperature fluctuation management is marked with a negative sign (-), it means that the coil temperature falls over time during the temperature fluctuation management.

[0026] Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet (decarburized annealed sheet) and dried. After that, finish annealing was performed at a maximum temperature of 1200°C to obtain a grain-oriented electrical steel sheet. During this finish annealing, a purification treatment was also performed by holding the steel sheet at 1150°C or higher in a hydrogen atmosphere for 10 hours.

[0027] For each grain-oriented electrical steel sheet thus obtained after the final annealing, the magnetic flux density B ８ The magnetic flux density (when a magnetic field of 800 A / m was applied) was measured, and the results are shown in FIG.

[0028] From FIG. 3, it was found that a grain-oriented electrical steel sheet with a high magnetic flux density can be obtained by controlling the temperature rise / fall rate in the temperature fluctuation control during the decarburization annealing process within the range of −20° C. / s to 20° C. / s.

[0029] <Experiment 4> A coil of cold-rolled sheet having a final thickness obtained under the same conditions as in Experiment 1 was used. This coil was stored at an average storage temperature of 50°C for a storage time of 200 hours after the end of the secondary cold rolling, which is the final cold rolling. ２ and N ２ In a mixed atmosphere, decarburization annealing, which also served as primary recrystallization annealing, was performed under the conditions of a holding temperature of 840°C and a holding time of 100 seconds. The temperature increase in this decarburization annealing began at a rate of 200°C / s, and during this temperature increase, temperature fluctuation control was performed at a temperature increase rate of 10°C / s, an average temperature of 150 to 420°C, and a time of 0.2 to 8.0 seconds. Thereafter, the temperature was again increased to 600°C at a rate of 200°C / s. The temperature was further increased to the above-mentioned holding temperature at a rate of 25°C / s, and after holding for the above-mentioned holding time, the sample was allowed to cool in the atmosphere.

[0030] Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet (decarburized annealed sheet) and dried. After that, finish annealing was performed at a maximum temperature of 1200°C to obtain a grain-oriented electrical steel sheet. During this finish annealing, a purification treatment was also performed by holding the steel sheet at 1150°C or higher in a hydrogen atmosphere for 10 hours.

[0031] For each grain-oriented electrical steel sheet thus obtained after the final annealing, the magnetic flux density B ８ The magnetic flux density (magnetic flux density when a magnetic field of 800 A / m was applied) was measured. The results are shown in Figure 4. In Figure 4, the magnetic flux density B of each grain-oriented electrical steel sheet subjected to temperature fluctuation management at different average temperatures T and times t is ８The values ​​are indicated on the graph as ◯ (good) when they are 1.928T or more, and as × (bad) when they are less than 1.928T.

[0032] It is clear from FIG. 4 that a grain-oriented electrical steel sheet with high magnetic flux density can be obtained when the average temperature T and time t in the temperature fluctuation control during the decarburization annealing process satisfy all of the following formulas (1) to (3).

[0033]

[0034] Although it is not clear why satisfying the above conditions results in a good magnetic flux density, the present inventors believe as follows: First, it is believed that the amount of dislocations fixed by interstitial elements such as C and N is optimized by aging when the above storage conditions, i.e., the predetermined average storage temperature and the predetermined storage time, are applied during storage of the cold-rolled sheet coil from the end of final cold rolling to the start of decarburization annealing.

[0035] The movement of elements is less likely to occur in pinned dislocations than in unpinned dislocations. Therefore, if there is a difference in dislocation density within a steel sheet (cold-rolled sheet), it will act as a driving force for the nucleation of primary recrystallization. Therefore, when there are many pinned dislocations, the pinned dislocations are present evenly throughout the entire steel sheet sample, and even if the above-mentioned temperature fluctuation control is performed in the recovery temperature range, the movement of elements does not occur sufficiently, making it difficult to create a difference in dislocation density within the steel sheet. In other words, it is necessary to appropriately control the amount of dislocations pinned by interstitial elements by aging.

[0036] Here, aging occurs during storage of the cold-rolled steel sheet coil from the end of final cold rolling to the start of decarburization annealing. By setting the average storage temperature and storage time during this storage under the above-mentioned predetermined conditions, the amount of fixed dislocations in the cold-rolled steel sheet can be controlled, and highly oriented Goss grains can be preferentially nucleated during nucleation of primary recrystallization. It is believed that such control of the amount of dislocations and nucleation improves grain orientation, thereby improving the magnetic flux density of the resulting grain-oriented electrical steel sheet.

[0037] Furthermore, it is believed that the magnetic flux density of the grain-oriented electrical steel sheet obtained was increased because appropriate dislocation movement occurred when the above-mentioned specified temperature fluctuation control involving a relatively slow temperature change with respect to time was carried out as a condition for decarburization annealing after the storage during the rapid heating process.

[0038] Even if the above-mentioned predetermined temperature fluctuation control is performed, it is believed that elements even in pinned dislocations may move if the temperature is high or for a long period of time. In this case, the movement of the pinned dislocations homogenizes the dislocation density distribution in the steel sheet sample as a whole, making it difficult to generate differences in dislocation density within the sample.

[0039] On the other hand, if the temperature fluctuation control described above is too low or too short, the movement of elements will not occur even in dislocations that are not pinned, even if temperature fluctuation control is performed. In other words, if the temperature fluctuation control is performed at a low temperature below the specified condition or for a short time, dislocation movement will not occur regardless of whether or not there is pinning, and dislocations will be present evenly throughout the steel sheet sample. As a result, the difference in dislocation density within the steel sheet is small, and the driving force for nucleation of primary recrystallization is small.

[0040] Therefore, it is necessary to appropriately control the dislocation movement due to recovery by managing the temperature fluctuations at an optimum average temperature T and time t that satisfy all of the above-mentioned formulas (1) to (3).

[0041] In the present invention, by performing such storage and temperature fluctuation management on the cold-rolled steel sheet, pinned dislocations do not move and unpinned dislocations recover, allowing for selective dislocation movement. That is, in the present invention, by performing coil storage and temperature fluctuation management under the above-mentioned predetermined conditions, it is possible to effectively control the dislocation density in the steel sheet, preferentially nucleate highly oriented Goss grains during nucleation of primary recrystallization, and improve the grain orientation. This is thought to result in an improved magnetic flux density of the resulting grain-oriented electrical steel sheet.

[0042] <Method for Manufacturing Grain-Oriented Electrical Steel Sheet> In the method for manufacturing a grain-orientated electrical steel sheet of the present invention, a steel material having a predetermined chemical composition is subjected to the following steps in order to obtain a grain-orientated electrical steel sheet: hot rolling, optional hot-rolled sheet annealing, cold rolling and optional intermediate annealing, decarburization annealing that also serves as primary recrystallization annealing, application of an annealing separator, and finish annealing. The manufacturing method of the present invention may further include other steps in addition to these basic steps. In carrying out these basic steps, it is essential to store the cold-rolled sheet coil at a predetermined average storage temperature and storage time after the end of the final cold rolling in the cold rolling step and before the start of the decarburization annealing step; and to control the temperature fluctuation of the cold-rolled sheet coil at a predetermined average temperature T, time t, and heating / cooling rate during the temperature rise process in the decarburization annealing step. By performing these storage treatments and temperature fluctuation control, a grain-orientated electrical steel sheet capable of exhibiting a high magnetic flux density can be manufactured, as described above.

[0043] [Composition] First, the composition of the steel material (steel slab) used in the manufacturing method of grain-oriented electrical steel sheet will be described. Note that "%" representing the content of the following elements means "% by mass" unless otherwise specified, and "ppm" representing the content of the following elements means "ppm by mass" unless otherwise specified.

[0044] [C: 0.002 to 0.100%] If the C content is less than 0.002%, the grain boundary strengthening effect of C is lost, resulting in defects that hinder manufacturing, such as cracks in the slab. On the other hand, if the C content exceeds 0.100%, it becomes difficult to reduce the C content to 0.005% or less, at which magnetic aging does not occur, by decarburization annealing. Therefore, the C content must be in the range of 0.002 to 0.100%. The C content is preferably 0.010% or more. Furthermore, the C content is preferably 0.080% or less.

[0045] [Si: 2.00 to 4.50%] Si is an element necessary for increasing the resistivity of steel and reducing iron loss. The above effects are not sufficient if the Si content is less than 2.00%. On the other hand, if the Si content exceeds 4.50%, workability decreases, making it difficult to manufacture by rolling. Therefore, the Si content must be in the range of 2.00 to 4.50%. The Si content is preferably 2.50% or more. Furthermore, the Si content is preferably 4.00% or less.

[0046] [Mn: 0.01 to 0.50%] Mn is an element necessary for improving the hot workability of steel. The above effect is not sufficient if the Mn content is less than 0.01%. On the other hand, if the Mn content exceeds 0.50%, the magnetic flux density of the grain-oriented electrical steel sheet as a finished sheet decreases. Therefore, the Mn content must be in the range of 0.01 to 0.50%. The Mn content is preferably 0.02% or more. Furthermore, the Mn content is preferably 0.20% or less.

[0047] The remainder of the steel material other than the basic elements C, Si and Mn can be Fe and unavoidable impurities.

[0048] In addition to the above-mentioned C, Si, and Mn, Al, N, S, and / or Se may be further contained to induce secondary recrystallization. The preferred contents of Al, N, S, and Se are divided into cases where an inhibitor is used and cases where it is not. First, we will discuss cases where an inhibitor is used to induce secondary recrystallization. For example, when an AlN-based inhibitor is used, it is preferable to contain Al and N in the ranges of Al: 0.005 to 0.050% and N: 0.0030 to 0.0200%, respectively. Furthermore, when an MnS / MnSe-based inhibitor is used, it is preferable to contain the above-mentioned amount of Mn and one or two of S: 0.002 to 0.030% and Se: 0.003 to 0.030%. If the amount of each element added is less than the above-mentioned lower limit, the inhibitor effect will not be sufficient. On the other hand, if the upper limit is exceeded, the inhibitor component remains undissolved when the slab is heated, resulting in a deterioration in magnetic properties. Note that AlN-based and MnS.MnSe-based inhibitors may be used in combination.

[0049] Next, we will discuss the case where no inhibitor is used to induce secondary recrystallization. In this case, it is preferable to reduce the contents of the inhibitor-forming elements Al, N, S, and Se as much as possible. Specifically, it is preferable to use a steel material in which the contents of Al, N, S, and Se are reduced to less than 0.005%, less than 0.0050%, less than 0.005%, and less than 0.003%, respectively.

[0050] Furthermore, for the purpose of improving magnetic properties, one or more elements selected from Group A, namely, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.010 to 0.500%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.500%, Sb: 0.005 to 0.500%, and Mo: 0.005 to 0.500%, may be added as appropriate.

[0051] In addition to or instead of the above, one or more elements selected from the B group, namely, B: 0.1 to 25.0 ppm, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and Co: 0.002 to 0.050%, may be added as appropriate.

[0052] Furthermore, in addition to or instead of the above, one or more elements selected from Group C, namely, As: 0.0010 to 0.0200%, Pb: 0.0001 to 0.0100%, W: 0.0010 to 0.0100%, and Zn: 0.001 to 0.020%, may be added as appropriate.

[0053] In addition to or instead of the above, one or more elements selected from Group D, namely Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ca: 0.001 to 0.020%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Nd: 0.001 to 0.020%, and La: 0.001 to 0.020%, may be added as appropriate.

[0054] The elements selected from Groups A to D may be added in duplicate as appropriate within the above-mentioned respective content ranges.

[0055] [Manufacturing Process] Next, each step in the manufacturing method of grain-oriented electrical steel sheet will be described. [Preparation of Steel Material] Steel having the above-mentioned composition can be produced by a conventional refining process, and then a steel material (steel slab) can be produced by a conventionally known ingot-making and blooming rolling method or continuous casting method. Alternatively, a steel material (thin cast piece) having a thickness of 100 mm or less can be produced from the produced steel by a direct casting method.

[0056] [Hot rolling] The steel material can be heated to about 1400°C in accordance with a conventional method, for example, if it contains an inhibitor component. On the other hand, if the steel material does not contain an inhibitor component, it can be heated to a temperature of 1250°C or lower. Next, the steel material is subjected to hot rolling in accordance with a conventional method to obtain a hot-rolled sheet. Note that if the steel material does not contain an inhibitor component, it may be hot-rolled immediately after casting without heating.

[0057] [Hot-rolled sheet annealing] The obtained hot-rolled sheet may be subjected to hot-rolled sheet annealing as necessary to form a hot-rolled annealed sheet. The annealing temperature for this hot-rolled sheet annealing is preferably in the range of 800 to 1150°C in order to obtain good magnetic properties. If the annealing temperature is less than 800°C, the band structure formed by hot rolling remains, making it difficult to obtain a granular primary recrystallized structure and inhibiting the development of secondary recrystallization. On the other hand, if the annealing temperature exceeds 1150°C, the grain size after hot-rolled sheet annealing becomes too coarse, making it difficult to obtain a granular primary recrystallized structure.

[0058] [Cold Rolling and Intermediate Annealing] The obtained hot-rolled sheet or hot-rolled annealed sheet is subjected to one cold rolling or two or more cold rollings sandwiched between intermediate annealings to obtain a cold-rolled sheet having the final sheet thickness. The annealing temperature for the intermediate annealing is preferably in the range of 900 to 1200°C. If the annealing temperature is less than 900°C, the recrystallized grains after the intermediate annealing tend to become finer, and furthermore, the Goss nuclei in the primary recrystallized structure tend to decrease, resulting in a decrease in the magnetic properties of the grain-oriented electrical steel sheet as a finished sheet. On the other hand, if the annealing temperature exceeds 1200°C, the crystal grains become too coarse, as in the case of hot-rolled sheet annealing, making it difficult to obtain a uniformly sized primary recrystallized structure.

[0059] Furthermore, in cold rolling to the final sheet thickness (final cold rolling), the steel sheet temperature during cold rolling is preferably increased to 100 to 300° C. Furthermore, in the final cold rolling, aging treatment is preferably performed once or multiple times at a steel sheet temperature of 100 to 300° C. during the cold rolling. These treatments are effective in improving the primary recrystallization texture and enhancing the magnetic properties of the grain-oriented electrical steel sheet.

[0060] [Storage of Coil] Here, in the present invention, it is important to store the coil obtained by winding the cold-rolled sheet so that the average storage temperature is 150°C or less and the storage time is 3 hours or more and 360 hours or less from the end of the final cold rolling to the start of the next process, decarburization annealing. The average storage temperature is preferably 120°C or less. From the viewpoint of cost, the average storage temperature is preferably 0°C or more. The storage time is preferably 4 hours or more. The storage time is preferably less than 360 hours. The end of the final cold rolling refers to the time when rolling is completed in the cold rolling process and the obtained cold-rolled sheet is wound into a coil. The start of the decarburization annealing refers to the moment when a sample (usually a cold-rolled sheet coil) is loaded into a furnace.

[0061] The average storage temperature is determined as long as the average temperature of the cold-rolled sheet coil from the end of the final cold rolling to the start of the decarburization annealing satisfies the above-mentioned predetermined range. Therefore, the temperature history during storage does not matter, such as one or more instantaneous temperature increases using pulse waves, multi-stage temperature increases, multi-stage cooling, gradual heating, gradual cooling, soaking, etc. The average storage temperature can be determined by constantly measuring the surface temperature history of the cold-rolled sheet coil during storage using a thermometer and automatically calculating the average temperature value.

[0062] [Decarburization annealing and temperature fluctuation control] Then, decarburization annealing, which also serves as primary recrystallization annealing, is performed to obtain a decarburized annealed sheet. From the viewpoint of decarburization, it is preferable that the annealing temperature for this decarburization annealing be set and maintained in the range of 800 to 900°C. Alternatively, the sheet may be subsequently cooled by natural air cooling. Furthermore, it is preferable that the atmosphere for the decarburization annealing be a mixed atmosphere of hydrogen and nitrogen and a humid atmosphere, from the viewpoint of favorably improving the controllability of the amount of decarburization. In addition, when nitriding the steel sheet (cold-rolled sheet), it is desirable to use a mixed gas of hydrogen, nitrogen, and ammonia.

[0063] What is important in the present invention is to provide temperature fluctuation management under the above-mentioned predetermined conditions in this decarburization annealing step. That is, during the temperature rise process during decarburization annealing, temperature rise (gradual heating), soaking, or cooling (slow cooling) is performed at a temperature rise or temperature fall rate of ±20°C / s or less under conditions of an average temperature T (°C) and time t (s) that satisfy the following formulas (1), (2), and (3): The temperature rise or temperature fall rate is preferably −15°C / s or more, more preferably −10°C / s or more, and is preferably 15°C / s or less, more preferably 10°C / s or less.

[0064]

[0065] Here, as long as the average temperature T in the temperature fluctuation control satisfies the above conditions, the temperature history during control does not matter, such as one or more instantaneous temperature increases or decreases using pulse waves, multi-stage temperature increases or decreases, soaking, etc. Furthermore, the temperature rise rate in the section from the coil temperature at the end of storage to 600°C is the temperature rise rate during the time excluding the time during which temperature fluctuation control is performed.

[0066] [Application of Annealing Separator and Finish Annealing] The decarburization-annealed sheet thus subjected to decarburization annealing is then coated with an annealing separator on one or both sides and subjected to finish annealing to produce a grain-oriented electrical steel sheet. When iron loss characteristics are important and a forsterite film is to be formed on the steel sheet, an annealing separator mainly composed of MgO can be applied to the surface of the steel sheet, dried, and then finish annealed. This facilitates the development of a secondary recrystallized structure highly concentrated in the Goss orientation during finish annealing and the formation of a forsterite film. On the other hand, when punching workability is important and the formation of a forsterite film is not desired, it is preferable to apply no annealing separator or to use an annealing separator mainly composed of silica or alumina, etc., and then perform finish annealing.

[0067] In addition, when a forsterite coating is not formed, it is also effective to apply the annealing separator by electrostatic application, which does not bring in moisture. Also, a heat-resistant inorganic material sheet (silica, alumina, mica) may be used as the annealing separator.

[0068] When a forsterite film is to be formed, the annealing temperature for the finish annealing is preferably 800°C or higher to induce secondary recrystallization. Furthermore, in order to complete the secondary recrystallization, it is preferable to hold the steel at a temperature of 800°C or higher for 15 hours or longer. On the other hand, when a forsterite film is not to be formed, it is sufficient to complete the secondary recrystallization, so the annealing temperature is preferably in the range of 850 to 950°C, and it is possible to complete the finish annealing by simply holding the steel in this temperature range for several hours or longer.

[0069] When a purification treatment is performed to emphasize iron loss characteristics and / or when a forsterite film is formed to reduce noise in a transformer, it is preferable to raise the annealing temperature in the final annealing to about 1200°C.

[0070] [Other Steps] After such finish annealing, the grain-oriented electrical steel sheet is subjected to water washing, brushing, pickling, etc. to remove unreacted annealing separator adhering to the steel sheet surface, and then flattening annealing to correct the shape is effective in reducing iron loss. This is because finish annealing is generally performed in a coiled state, which can cause the coil to develop a curl, which can cause deterioration of characteristics during iron loss measurement.

[0071] Furthermore, when grain-oriented electrical steel sheets are used in a stack, it is effective to apply an insulating coating to the surface of the steel sheet before or after the flattening annealing. In particular, in order to reduce iron loss, it is preferable to apply a tension-applying coating that can apply tension to the steel sheet as the insulating coating. Note that, for forming the tension-applying coating, it is preferable to adopt a method of applying a tension coating via a binder or a method of depositing an inorganic substance on the surface layer of the steel sheet by physical vapor deposition or chemical vapor deposition, because this makes it possible to form an insulating coating that has excellent coating adhesion and a significant iron loss reduction effect.

[0072] In order to further reduce iron loss, it is preferable to carry out a magnetic domain refining treatment. Examples of the treatment method that can be used include a commonly used method of forming grooves in a final product sheet, a method of introducing linear or point-shaped thermal strain and / or impact strain by laser irradiation, electron beam irradiation, or plasma irradiation, and a method of forming grooves by etching the surface of a steel sheet in an intermediate process, such as a cold-rolled sheet that has been cold-rolled to the final thickness.

[0073] In the method for producing grain-oriented electrical steel sheets, any items not described in this specification can be produced by conventional methods.

[0074] [Example 1] A steel slab consisting of 0.070% C, 3.35% Si, 0.10% Mn, 0.025% Al, 0.025% Se, 0.0120% N, and the balance Fe and unavoidable impurities was produced by continuous casting. The steel slab was heated to a temperature of 1420 ° C and then hot-rolled to a hot-rolled sheet having a thickness of 2.6 mm. This hot-rolled sheet was subjected to hot-rolled sheet annealing under conditions of 1050 ° C × 40 seconds to obtain a hot-rolled annealed sheet. This hot-rolled annealed sheet was subjected to primary cold rolling to reduce the intermediate thickness to 1.9 mm, intermediate annealing under conditions of 1050 ° C × 30 seconds, and then secondary cold rolling as final cold rolling to obtain a cold-rolled sheet having a final thickness of 0.23 mm. The finished cold-rolled sheet was wound into a coil.

[0075] The cold-rolled coils were stored from the end of the final cold rolling to the final thickness until the start of decarburization annealing. The storage conditions at this time were the average storage temperature (°C) and storage time (h, hours) as shown in Table 1. The coils after each storage were subjected to H ２ and N ２ In a mixed atmosphere of 840°C and 100 seconds, decarburization annealing, which also served as primary recrystallization annealing, was performed under the conditions of a holding temperature of 840°C and a holding time of 100 seconds. The temperature increase from the coil temperature after storage in this decarburization annealing began at 200°C / s, and during this temperature increase, temperature fluctuation control was performed under the conditions of the average temperature T (°C), time t (s, seconds), and temperature increase / decrease rates (°C / s) shown in Table 1. The temperature was then increased to 600°C at a temperature increase rate of 200°C / s. The temperature was further increased to the above-mentioned holding temperature at a rate of 25°C / s, and after holding for the above-mentioned holding time, the specimen was allowed to cool in air.

[0076] Next, an annealing separator mainly composed of MgO was applied to the surface of the decarburized annealed sheet and dried. After that, finish annealing was performed at a maximum temperature of 1200°C to obtain a grain-oriented electrical steel sheet. During this finish annealing, a purification treatment was also performed by holding the sheet at 1150°C or higher in a hydrogen atmosphere for 10 hours.

[0077] For each grain-oriented electrical steel sheet after the final annealing, the magnetic flux density B when a magnetic field of 800 A / m was applied using the method described in JIS C2556 was measured. ８The results are shown in Table 1. In Table 1, the left side of formula (1) is shown as the A value, as shown in formula (4) below.

[0078]

[0079]

[0080] From Table 1, it can be seen that by following the manufacturing method of the present invention, a grain-oriented electrical steel sheet with high magnetic flux density can be obtained.

[0081] [Example 2] A steel slab having the chemical composition shown in Table 2, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. This steel slab was heated to a temperature of 1200°C and then hot-rolled to form a hot-rolled sheet having a thickness of 2.3 mm. This hot-rolled sheet was subjected to hot-rolled sheet annealing under the conditions of 1030°C x 50 seconds to form a hot-rolled annealed sheet. This hot-rolled annealed sheet was cold-rolled as a final cold rolling to form a cold-rolled sheet having a final thickness of 0.23 mm. The finished cold-rolled sheet was wound into a coil.

[0082] The cold-rolled coils were stored from the end of the final cold rolling to the final thickness until the start of decarburization annealing. The storage conditions were an average storage temperature of 30°C and a storage time of 120 hours. ２ and N ２ The steel sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, under the conditions of a holding temperature of 830°C and a holding time of 120 seconds in a mixed atmosphere of H. ２ , N ２ and N.H. ３ The steel sheet was subjected to additional nitriding treatment in a mixed atmosphere of 1000 ppm nitrogen and 1000 ppm ammonium hydroxide, so that the nitrogen concentration of the steel sheet became 300 ppm.

[0083] The temperature increase from the coil temperature after storage in the decarburization annealing process started at 200°C / s, and during this temperature increase, temperature fluctuation control was performed at a temperature decrease rate of -5°C / s to an average temperature T of 290°C and a time t of 1.0 s. Thereafter, the temperature was increased to 600°C at a temperature increase rate of 200°C / s. The temperature was further increased to the holding temperature at a rate of 25°C / s, and after holding for the holding time, the steel was allowed to cool in the atmosphere.

[0084] Next, an annealing separator mainly composed of MgO was applied to the surface of the decarburized annealed sheet and dried. After that, finish annealing was performed at a maximum temperature of 1200°C to obtain a grain-oriented electrical steel sheet. During this finish annealing, a purification treatment was also performed by holding the sheet at 1150°C or higher in a hydrogen atmosphere for 20 hours.

[0085] For each grain-oriented electrical steel sheet thus obtained after the final annealing, the magnetic flux density B ８ The magnetic flux density (when a magnetic field of 800 A / m was applied) was measured. The results are shown in Table 2.

[0086]

[0087] From Table 2, it can be seen that by following the manufacturing method of the present invention, a grain-oriented electrical steel sheet with high magnetic flux density can be obtained.

Claims

1. A method for producing a grain-oriented electrical steel sheet, comprising: hot rolling a steel material containing, by mass, 0.002 to 0.100% C, 2.00 to 4.50% Si, and 0.01 to 0.50% Mn, with the balance being Fe and unavoidable impurities, to form a hot-rolled sheet; annealing the hot-rolled sheet to form a hot-rolled annealed sheet, or, without annealing the hot-rolled sheet, cold rolling the hot-rolled sheet or the hot-rolled annealed sheet once or at least twice with intermediate annealing between them to form a cold-rolled sheet having a final thickness; decarburization annealing that also serves as primary recrystallization annealing to form a decarburization-annealed sheet; applying an annealing separator to the surface of the decarburization-annealed sheet, and then finish-annealing the decarburization-annealed sheet to obtain a grain-oriented electrical steel sheet, a method for producing a grain-oriented electrical steel sheet, wherein, during the period from the end of final cold rolling in the cold rolling to the start of decarburization annealing, a coil obtained by winding the cold-rolled sheet is stored at an average storage temperature of 150°C or less for a storage time of 3 to 360 hours; and during the temperature-raising process of the decarburization annealing, temperature fluctuation is controlled in a range of -20 to +20°C / s within a range of an average temperature T (°C) and a time t (s) that satisfy the following formulas (1) to (3):

2. A method for producing grain-oriented electrical steel sheet according to claim 1, wherein the steel material further contains, in mass%, one or more elements selected from Al: 0.005 to 0.050%, N: 0.0030 to 0.0200%, Se: 0.003 to 0.030%, and S: 0.002 to 0.030%.

3. The method for producing grain-oriented electrical steel sheet according to claim 1 or 2, wherein the steel material further contains one or more elements selected from at least one of the following groups A to D: Group A: In mass%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.010 to 0.500%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.500%, Sb: 0.005 to 0.500%, and Mo: 0.005 to 0.500%. Group B: In mass% or mass ppm, B: 0.1 to 25.0 ppm, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and Co: 0.002 to 0.050%. Group C: Group D: By mass %, As: 0.0010 to 0.0200%, Pb: 0.0001 to 0.0100%, W: 0.0010 to 0.0100%, and Zn: 0.001 to 0.020%. Group D: By mass %, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ca: 0.001 to 0.020%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Nd: 0.001 to 0.020%, and La: 0.001 to 0.020%.

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