Method for producing grain-oriented electrical steel sheet

Abstract

Provided is a method for producing a grain-oriented electrical steel sheet with which it is possible to prevent the breakage of the steel sheet in final cold rolling and that has fine Goss grains after secondary recrystallization annealing, and in which iron loss is low. A steel sheet having a predetermined component composition and subjected to hot-rolled sheet annealing or final intermediate annealing is subjected to aging treatment of maintaining, for 24-150 hours, a state in which the temperature of the steel sheet is 30-150°C, and then subjected to final cold rolling to obtain a cold-rolled sheet. In a temperature rising process when performing decarburization annealing on the cold-rolled sheet, holding treatment is performed in which the temperature increase / decrease speed of the cold-rolled sheet is set to -5°C / s to 25°C / s. The holding time t(s) and the holding temperature T(°C) satisfy all mathematical expressions of T ≤ 1500t -800, 400 ≤ T ≤ 700, and t ≤ 8.0.

Description

Manufacturing method of grain-oriented electrical steel sheet

[0001] The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet.

[0002] Grain-oriented electrical steel sheets are soft magnetic materials primarily used as transformer core materials. Grain-oriented electrical steel sheets have 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. The finer the grains, the lower the iron loss. Patent Document 1, for example, describes a method for refining grains in grain-oriented electrical steel sheets to reduce iron loss. It states that a holding treatment, in which the temperature of the steel sheet is maintained at a predetermined temperature for a certain period of time during the heating process during decarburization annealing, which also serves as primary recrystallization annealing, is effective. According to this method, the holding treatment during decarburization annealing homogenizes the temperature inside the steel sheet, preferentially recovering the <111> / / ND orientation, reducing the <111> / / ND orientation after primary recrystallization and increasing Goss nuclei. This results in finer recrystallized grains after secondary recrystallization, resulting in a grain-oriented electrical steel sheet with low iron loss. Here, ND refers to the direction normal to the surface of the steel sheet.

[0003] International Publication No. 2014 / 017589

[0004] In the general manufacturing process of grain-oriented electrical steel sheets, Goss grains with a {110}<001> orientation grow significantly from the primary recrystallized grains due to secondary recrystallization. The coarsening of secondary recrystallized grains increases eddy current loss, which in turn causes increased iron loss. Furthermore, coarsening of secondary recrystallized grains increases the likelihood of the steel sheet breaking during the final cold rolling process, a problem that needed to be addressed.

[0005] In view of the above problems, an object of the present invention is to provide a method for producing a grain-oriented electrical steel sheet that can prevent breakage of the steel sheet during final cold rolling and that can produce a grain-oriented electrical steel sheet with low iron loss in which Goss grains are refined after secondary recrystallization annealing.

[0006] The inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that by subjecting a steel sheet that has been annealed immediately before final cold rolling to an aging treatment, then subjecting the steel sheet to final cold rolling, and then subjecting the cold-rolled sheet after final cold rolling to a holding treatment under specific conditions, secondary recrystallized grains can be made finer, and iron loss of the grain-oriented electrical steel sheet can be further reduced compared to conventional techniques, which led to the completion of the present invention.

[0007] The gist and configuration of the present invention are as follows.

[0008] [1] A method for producing a grain-oriented electrical steel sheet, the method comprising the steps of hot rolling a steel material having a component composition containing, by mass%, C: 0.002 to 0.100%, Si: 2.0 to 4.5%, Mn: 0.005 to 0.50%, with the balance being Fe and unavoidable impurities, to obtain a hot-rolled sheet, annealing the hot-rolled sheet, thereafter cold rolling the hot-rolled sheet once or at least two times with intermediate annealing in between to obtain 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 obtain a decarburization-annealed sheet, applying an annealing separator to a surface of the decarburization-annealed sheet, and subjecting the decarburization-annealed sheet to finish annealing, a holding time t (s) of the cold-rolled sheet during the decarburization annealing step, and a holding temperature T (°C) of the cold-rolled sheet during the temperature rise step, wherein the cold-rolled sheet is subjected to a holding treatment in which the temperature rise / fall rate of the cold-rolled sheet is set to be -5°C / s or more and 25°C / s or less, and wherein the holding time t and the holding temperature T satisfy all of the following formulas (1) to (3): T≦1500t-800 (1) 400≦T≦700 (2) t≦8.0 (3)

[0009] [2] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein the temperature rising rate of the cold-rolled sheet in the temperature rising process excluding the holding treatment during the temperature rising process when the cold-rolled sheet is subjected to the decarburization annealing is 40°C / s or more.

[0010] [3] The component composition is: Group A: in mass%, one or more selected from Al: 0.005 to 0.050%, N: 0.003 to 0.020%, S: 0.002 to 0.030%, Se: 0.003 to 0.030%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, 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: one or more elements selected from, 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: one or more elements selected from, in 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: the method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], further comprising, in mass%, at least one element selected from 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%.

[0011] According to the present invention, it is possible to prevent breakage of the steel sheet during final cold rolling, and to produce a grain-oriented electrical steel sheet with low iron loss in which Goss grains are refined after secondary recrystallization annealing.

[0012] 1 is a graph showing the relationship between the time of aging treatment and iron loss; 2 is a graph showing the relationship between the temperature of aging treatment and iron loss; 3 is a graph showing the relationship between the temperature rise and fall rate of holding treatment in the temperature rise process of decarburization annealing and iron loss; and 4 is a graph showing the range of holding time t and holding temperature T in holding treatment.

[0013] First, the experiment that led to the invention will be described.

[0014] <Experiment 1> A steel material containing, by mass%, 0.068% C, 3.31% Si, 0.07% Mn, 0.016% Se, and 0.006% S, with the balance being Fe and unavoidable impurities, was produced by continuous casting. The obtained steel material was heated to a temperature of 1380 ° C. and then hot-rolled to obtain a hot-rolled sheet having a thickness of 2.5 mm. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing at 1010 ° C. for 60 seconds (seconds), then subjected to primary cold rolling to obtain an intermediate thickness of 1.7 mm, and then subjected to intermediate annealing at 1050 ° C. for 25 seconds to obtain a plurality of steel sheets. The obtained steel sheet was charged into a heat-retaining furnace controlled at a temperature of 70 ° C., and subjected to aging treatment under 19 time conditions ranging from 10 hours (hours) to 250 hours. After that, final cold rolling was performed to obtain a cold-rolled sheet having a final thickness of 0.23 mm. Next, the obtained cold-rolled sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, in a mixed gas atmosphere of hydrogen and nitrogen. In the temperature-raising process of decarburization annealing, the temperature was first raised using a radiant heating furnace under conditions such that the temperature rise rate of the cold-rolled sheet was 50 ° C. / s. Next, the output of the radiant heating furnace was adjusted during the temperature-raising process to a temperature rise / decrease rate of ±0 ° C. / s, and the cold-rolled sheet was subjected to a holding treatment with a holding temperature of 540 ° C. and a holding time of 3.0 s. Next, the cold-rolled sheet was heated to 720 ° C. at a heating rate of 150 ° C. / s using an induction heating furnace, and then heated to 830 ° C. at a heating rate of 40 ° C. / s using a radiant heating furnace again, and decarburization annealing was performed at 830 ° C. for 100 s. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet, and after the annealing separator was dried, the steel sheet was subjected to finish annealing for 10 hours in a hydrogen atmosphere at a maximum temperature of 1220°C, to obtain 19 types of grain-oriented electrical steel sheets with different aging treatment times. The iron loss W of the obtained grain-oriented electrical steel sheets at a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz was measured by the method described in Japanese Industrial Standard JIS C 2556. １７／５０ The results are shown in Figure 1. Figure 1 shows that when the aging temperature is 70°C and the aging time is 24 hours or more and 150 hours or less, a grain-oriented electrical steel sheet with low iron loss can be obtained.

[0015] <Experiment 2> The steel sheet subjected to intermediate annealing obtained in Experiment 1 was charged into a heat-retention furnace controlled at 19 temperatures from 5 ° C. to 250 ° C., and subjected to aging treatment for 100 hours. After that, final cold rolling was performed to produce a cold-rolled sheet with a final thickness of 0.23 mm. Next, the obtained cold-rolled sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, in a mixed gas atmosphere of hydrogen and nitrogen. During the temperature rise process of the decarburization annealing, the cold-rolled sheet was first heated using an induction heating furnace under conditions such that the temperature rise rate was 150 ° C. / s. Next, during the temperature rise process, a radiant heating furnace was used to heat the cold-rolled sheet at a temperature rise / decrease rate of 10 ° C. / s, and the cold-rolled sheet was subjected to a holding treatment at 600 ° C. for a holding time of 3.0 s. Next, the cold-rolled sheets were heated to 720°C at a heating rate of 150°C / s using an induction heating furnace, and then again to 840°C at a heating rate of 45°C / s using a radiant heating furnace, where they were subjected to decarburization annealing at 840°C for 110 seconds. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet, and after the annealing separator was dried, the steel sheet was subjected to finish annealing for 12 hours in a hydrogen atmosphere at a maximum temperature of 1200°C, thereby obtaining 19 types of grain-oriented electrical steel sheets with different aging treatment temperatures. The iron loss W of the obtained grain-oriented electrical steel sheets was measured using the same method as in Experiment 1. １７／５０ The results are shown in Figure 2. Figure 2 shows that when the aging treatment time is 100 hours and the aging treatment temperature is 30°C or higher and 150°C or lower, a grain-oriented electrical steel sheet with low iron loss can be obtained.

[0016] <Experiment 3> The steel sheet subjected to intermediate annealing obtained in Experiment 1 was loaded into a heat-retention furnace controlled at a temperature of 50 ° C., subjected to aging treatment for 100 hours, and then subjected to final cold rolling to produce a cold-rolled sheet with a final thickness of 0.23 mm. Next, decarburization annealing, which also served as primary recrystallization annealing, was performed in a mixed gas atmosphere of hydrogen and nitrogen. During the temperature rise process of decarburization annealing, the cold-rolled sheet was first heated using an induction heating furnace under conditions where the temperature rise rate was 70 ° C. / s. Next, during the temperature rise process, a radiant heating furnace and cooling by atmospheric gas injection were used to heat the cold-rolled sheet under 19 conditions ranging from -10 ° C. / s to 30 ° C. / s, and the cold-rolled sheet was subjected to a holding treatment with a holding temperature of 530 ° C. and a holding time of 5.0 s. Here, the holding temperature of 530 ° C. is an intermediate temperature between the temperature of the cold-rolled sheet at the start and end of the holding time of 5.0 s. Next, the cold-rolled sheet was heated to 720°C at a heating rate of 150°C / s using an induction heating furnace, and then heated to 810°C at a heating rate of 50°C / s using a radiant heating furnace again, and decarburization annealing was performed at 810°C for 150 seconds. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet, and after the annealing separator was dried, the steel sheet was subjected to finish annealing for 11 hours in a hydrogen atmosphere at a maximum temperature of 1220°C, thereby obtaining 19 types of grain-oriented electrical steel sheets with different heating and cooling rates during the holding treatment. The iron loss W of the obtained grain-oriented electrical steel sheets was measured using the same method as in Experiment 1. １７／５０ The results are shown in Figure 3. Figure 3 shows that when the holding temperature in the holding treatment is 530°C and the holding time is 5.0 seconds, a grain-oriented electrical steel sheet with low iron loss can be obtained when the temperature increase / decrease rate in the holding treatment is -5°C / s or more and 25°C / s or less.

[0017] <Experiment 4> The steel sheet subjected to intermediate annealing obtained in Experiment 1 was loaded into a heat-retention furnace controlled at a temperature of 80 ° C., subjected to aging treatment for 50 hours, and then subjected to final cold rolling to produce a cold-rolled sheet with a final thickness of 0.23 mm. Next, decarburization annealing, which also served as primary recrystallization annealing, was performed in a mixed gas atmosphere of hydrogen and nitrogen. During the temperature rise process of decarburization annealing, the cold-rolled sheet was first heated using an induction heating furnace under conditions where the temperature rise rate was 250 ° C. / s. Next, during the temperature rise process, a radiant heating furnace was used to heat the cold-rolled sheet at a temperature rise / decrease rate of 15 ° C. / s. The cold-rolled sheet was subjected to a holding treatment in 26 combinations ranging from 380 ° C. to 720 ° C. and a holding time ranging from 0.6 s to 10.0 s. Here, the holding temperature was an intermediate temperature between the temperature of the cold-rolled sheet at the start and end of the holding time. Next, for samples in which the holding temperature of the holding treatment was less than 720°C, the cold-rolled sheet was heated to 720°C at a heating rate of 150°C / s using an induction heating furnace. Next, for all samples, the temperature was raised to 870°C at a heating rate of 40°C / s using a radiant heating furnace, and decarburization annealing was performed at 870°C for 100 seconds. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet, and after drying the annealing separator, the steel sheet was subjected to finish annealing for 10 hours in a hydrogen atmosphere at a maximum temperature of 1230°C, thereby obtaining 26 types of grain-oriented electrical steel sheets with different holding temperatures and holding times in the holding treatment. The iron loss W of the obtained grain-oriented electrical steel sheets was measured using the same method as in Experiment 1. １７／５０ The results are shown in Figure 4. In Figure 4, circles represent the measured iron loss W １７／５０ indicates that the iron loss W is 0.830 W / kg or less. １７／５０ It can be seen from Figure 4 that when the temperature rise / fall rate during the holding treatment is 15°C / s, and the holding time is t (s) and the holding temperature is T (°C), a grain-oriented electrical steel sheet with low iron loss can be obtained in the specified ranges that satisfy all of the following formulas (1) to (3): T≦1500t-800 (1) 400≦T≦700 (2) t≦8.0 (3)

[0018] Next, an embodiment of the present invention will be described.

[0019] In one embodiment, the present invention provides a method for producing a grain-oriented electrical steel sheet, the method comprising the steps of hot rolling a steel material having a component composition containing, by mass%, C: 0.002 to 0.100%, Si: 2.0 to 4.5%, Mn: 0.005 to 0.50%, with the balance being Fe and unavoidable impurities, to obtain a hot-rolled sheet, annealing the hot-rolled sheet, subsequently cold rolling the hot-rolled sheet once or two or more times with intermediate annealing in between to obtain 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 obtain a decarburization-annealed sheet, applying an annealing separator to the surface of the decarburization-annealed sheet, and finish-annealing the decarburization-annealed sheet, The present invention relates to a method for producing a grain-oriented electrical steel sheet, characterized in that (I) when cold rolling is performed once, after hot-rolled sheet annealing and before cold rolling, or (II) when cold rolling is performed two or more times, after final intermediate annealing performed immediately before final cold rolling and before final cold rolling, the steel sheet is subjected to an aging treatment in which the temperature of the steel sheet is maintained at 30°C or higher and 150°C or lower for 24 hours or longer and 150 hours or shorter, and during the temperature rise process when the cold-rolled sheet is subjected to the decarburization annealing, the cold-rolled sheet is subjected to a holding treatment in which the temperature rise / fall rate of the cold-rolled sheet is -5°C / s or higher and 25°C / s or lower, and the holding time t and the holding temperature T satisfy all of the following mathematical formulas (1) to (3): T≦1500t-800 (1) 400≦T≦700 (2) t≦8.0 (3)

[0020] 1. Composition The composition of the steel material (slab) of the grain-oriented electrical steel sheet to which the present invention is applied will be described. In this specification, the composition of the steel material is expressed in mass percentage (mass%) or parts per million (mass ppm).

[0021] 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 point magnetic aging does not occur, by decarburization annealing. Therefore, C is set to the range of 0.002 to 0.100%. Preferably, C is set to the range of 0.010 to 0.080%.

[0022] Si: 2.0 to 4.5% Si is an element necessary for increasing the resistivity of steel and reducing iron loss. The above effects are insufficient at less than 2.0%, while at more than 4.5%, workability decreases, making it difficult to manufacture by rolling. Therefore, the Si content is set to the range of 2.0 to 4.5%. Preferably, the S content is set to the range of 2.5 to 4.0%.

[0023] Mn: 0.005 to 0.50% Mn is an element necessary for improving the hot workability of steel. The above effect is insufficient at a content of less than 0.005%, while at a content of more than 0.50%, the magnetic flux density of the finished sheet decreases. Therefore, Mn is set to the range of 0.005 to 0.50%. Preferably, Mn is set to the range of 0.02 to 0.20%. The above-mentioned C, Si, and Mn are essential elements contained in the steel material used to manufacture the grain-oriented electrical steel sheet according to the present invention.

[0024] The steel material has a composition with the balance being Fe and inevitable impurities. Here, inevitable impurities refer to impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing facilities, etc., and are allowed to be contained to an extent that does not impair the object of the present invention. Examples of raw materials containing inevitable impurities include iron ore, reduced iron, and scrap.

[0025] In another embodiment, the component composition is: Group A: in mass %, one or more selected from Al: 0.005 to 0.050%, N: 0.003 to 0.020%, S: 0.002 to 0.030%, Se: 0.003 to 0.030%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, 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: one or more elements selected from, 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: one or more elements selected from, in 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: Further containing, in mass %, at least one element selected from 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%. These elements are optional elements contained in the steel material in addition to the above essential elements.

[0026] The optional elements described above will be explained separately for cases where an inhibitor for secondary recrystallization is used and cases where it is not. First, when an inhibitor for secondary recrystallization is used, when an AlN-based inhibitor is used, it is preferable to contain Al and N in the ranges of Al: 0.005-0.050% and N: 0.003-0.020%, respectively. Furthermore, when an MnS / MnSe-based inhibitor is used, it is preferable to contain the aforementioned amount of Mn and one or two of S: 0.002-0.030% and Se: 0.003-0.030%. If the amount of each is less than the above-mentioned lower limit, the inhibitor effect is insufficient. On the other hand, if the amount added exceeds the above-mentioned upper limit, the inhibitor component remains undissolved during slab heating, resulting in a deterioration of magnetic properties. Note that AlN-based and MnS / MnSe-based inhibitors may be used simultaneously.

[0027] On the other hand, when an inhibitor is not used to induce secondary recrystallization, 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 reduce the contents of these elements contained in the steel material to Al: less than 0.005%, N: less than 0.0050%, S: less than 0.0050%, and Se: less than 0.0030%.

[0028] However, regardless of whether an inhibitor is used or not, for the purpose of improving magnetic properties, the steel material may further contain one or more optional elements selected from Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, 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%. All of the optional elements described above belong to Group A.

[0029] Similarly, for the purpose of improving magnetic properties, the steel material contains one or more elements selected from 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% as group B, and As: 0.0010 to 0.0200%, Pb: 0.0001 to 0.0100%, W: 0.0010 to 0.0100% as group C. %, and Zn: 0.001 to 0.020% as Group D, and one or more elements selected from 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. In another embodiment, the chemical composition of the steel material further includes at least one of the above-mentioned Groups A to D in addition to the essential elements. Any combination of Groups A to D may be used.

[0030] 2. Manufacturing Method Next, a method for manufacturing the grain-oriented electrical steel sheet according to the present invention will be described. After a steel having the above-described composition is melted by a conventional refining process, a slab obtained by a conventionally known ingot-making and blooming rolling method or continuous casting method can be used as the steel material of the present invention. Alternatively, a thin cast slab having a thickness of 100 mm or less obtained by a direct casting method can also be used as the steel material. The steel material is heated according to a conventional method, for example, to about 1400°C if it contains an inhibitor component, or to a temperature of 1250°C or less if it does not contain an inhibitor component, and then hot-rolled to form a hot-rolled sheet. Note that if it does not contain an inhibitor component, it may be hot-rolled immediately after casting without heating.

[0031] Next, the obtained hot-rolled sheet is subjected to hot-rolled sheet annealing. 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.

[0032] Next, the hot-rolled sheet after the hot-rolled sheet annealing is subjected to one cold rolling or two or more cold rollings with intermediate annealing in between to obtain a cold-rolled sheet of 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. 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.

[0033] Here, (I) when cold rolling is performed once, after hot-rolled sheet annealing and before cold rolling, or (II) when cold rolling is performed twice or more, after final intermediate annealing performed immediately before final cold rolling and before final cold rolling, aging treatment is performed in which the temperature of the steel sheet is maintained at 30°C or higher and 150°C or lower for 24 hours or more and 150 hours or less, and then cold rolling is performed to obtain a cold-rolled sheet.

[0034] It is important to subject the steel sheet to aging treatment in which the temperature is maintained at 30°C or higher and 150°C or lower for 24 hours or longer and 150 hours or shorter from the completion of hot-rolled sheet annealing or final intermediate annealing until the start of cold rolling or final cold rolling. In this case, the start of the aging treatment time is defined as when the temperature of the steel sheet reaches 30°C or higher and 150°C or lower. The end of the aging treatment time is defined as when the temperature of the steel sheet falls outside the range of 30°C or higher and 150°C or lower. However, if final cold rolling is started while the temperature of the steel sheet is 30°C or higher and 150°C or lower, the end of the aging treatment time is defined as when final cold rolling is started. The temperature of the steel sheet can be monitored by known means.

[0035] If the aging temperature is less than 30°C, the temperature of the steel sheet at the time of biting in the final cold rolling is low, which can cause fracture. Furthermore, C and N segregate instead of diffusing, which tends to make it difficult for shear bands to form uniformly in the steel during the final cold rolling. On the other hand, if the aging temperature exceeds 150°C, the mobility of C and N increases, which can lead to segregation to grain boundaries, making it difficult for shear bands to form uniformly in the steel during cold rolling. If the aging time is less than 24 hours, the time is too short for C and N to disperse in the steel by diffusion, and if it exceeds 150 hours, the diffusion time is too long, which tends to lead to grain boundary segregation.

[0036] The above-mentioned aging treatment of the hot-rolled sheet can be carried out in the annealing furnace after the hot-rolled sheet annealing or the final intermediate annealing is completed.

[0037] If it is difficult to satisfy the above-mentioned temperature and time conditions for the aging treatment inside an annealing furnace, the steel sheet may be aged by coiling the steel sheet after hot-rolled sheet annealing or final intermediate annealing into a coil and keeping the coil at a temperature in a temperature-controllable heat-retention furnace. In this case, the start of the aging treatment time is determined to be when the surface temperature of the coil reaches 30°C or higher and 150°C or lower. The end of the aging treatment time is determined to be when the surface temperature of the coil falls outside the range of 30°C or higher and 150°C or lower. However, if final cold rolling is started while the surface temperature of the coil is 30°C or higher and 150°C or lower, the end of the aging treatment time is determined to be the moment the coil is unloaded when final cold rolling is started. The surface temperature of the coil can be monitored by known means.

[0038] In the final cold rolling, heat treatment in which the steel sheet is heated to a temperature of 100°C or higher and 300°C or lower is carried out once or multiple times during the cold rolling, which is effective in improving the primary recrystallization texture and enhancing the magnetic properties.

[0039] Next, the cold-rolled sheet is subjected to decarburization annealing, which also serves as primary recrystallization annealing. From the viewpoint of decarburization, the annealing temperature for this decarburization annealing is preferably in the range of 800°C or higher and 900°C or lower. Furthermore, the decarburization annealing atmosphere is preferably a wet atmosphere containing a mixed gas of hydrogen and nitrogen, since this makes it easier to control the amount of decarburization. When nitriding the steel sheet, a mixed gas atmosphere of hydrogen, nitrogen, and ammonia is preferably used. However, this does not apply when the steel material contains only 0.005% or less C and decarburization is not required. Primary recrystallization annealing may be performed separately from decarburization annealing.

[0040] What is important in decarburization annealing is that, in the temperature rising process when decarburization annealing is performed on a cold-rolled sheet, a holding treatment is performed at a temperature rising / falling rate of -5°C / s or more and 25°C / s or less, and when the time from the start to the end of the holding treatment is the holding time t (s) and the average temperature of the cold-rolled sheet during the holding treatment is the holding temperature T (°C), the holding time t and the holding temperature T must satisfy all of the following formulas (1) to (3): T≦1500t−800 (1) 400≦T≦700 (2) t≦8.0 (3)

[0041] Here, the start of the holding time t is defined as the time when the temperature increase rate of the cold-rolled sheet is slowed down from the high rate at the start of the temperature increase to a temperature increase / decrease rate of −5° C. / s or more and 25° C. / s or less, which is set as the temperature increase / decrease rate in the holding treatment. The end of the holding time t is defined as the time when the temperature increase rate of the cold-rolled sheet is accelerated from the certain temperature increase / decrease rate, which is set as the temperature increase / decrease rate in the holding treatment, to return to the initial high rate.

[0042] The holding temperature T is the time average value of the temperature of the cold-rolled sheet during the holding time t from the start to the end. If the temperature of the cold-rolled sheet hardly changes during the holding time t, that temperature may be regarded as the holding temperature T. The temperature of the cold-rolled sheet can be monitored by known means.

[0043] When the temperature increase / decrease rate of the cold-rolled sheet during the holding treatment is slower than -5°C / s (when the temperature decrease rate is faster than 5°C / s), the iron loss increases. When the temperature increase / decrease rate is faster than 25°C / s, the iron loss also increases.

[0044] The temperature increase / decrease rate of the cold-rolled sheet during the holding treatment can be controlled within the above range, for example, by controlling the output of a radiant heating furnace or an induction heating furnace during the temperature increase, and by controlling the injection of atmospheric gas during the temperature decrease. Here, as long as the holding time t and the holding temperature T satisfy the above conditions, the temperature history of the cold-rolled sheet is not important. The temperature history of the cold-rolled sheet may be, for example, one or more instantaneous temperature increases using pulse waves, multi-stage temperature increases, multi-stage cooling, gradual heating, gradual cooling, soaking, etc.

[0045] The reason why the grain-oriented electrical steel sheet that was subjected to the aging treatment and holding treatment under the above conditions during the manufacturing process has low iron loss is not clearly understood, but the inventors believe that this is because the grain size of secondary recrystallized grains in the grain-oriented electrical steel sheet is refined by the mechanism described below.

[0046] First, by applying the above-mentioned aging treatment conditions, the C and N contained in the steel sheet can be kept dispersed without segregation. By performing final cold rolling in a state where C and N are dispersed, the number of shear bands increases. Since Goss grains originate from shear bands, the number of Goss grains increases by subjecting the hot-rolled sheet to aging treatment under the above-mentioned conditions.

[0047] Next, when decarburization annealing, which also serves as primary recrystallization annealing, is performed under the above conditions, recovery of the cold-rolled sheet occurs, and most dislocations are eliminated in areas other than the shear bands. However, some dislocations remain in the shear bands, where many dislocations are present. This results in a difference in dislocation density between the shear bands and non-shear bands. The difference in dislocation density serves as the driving force for nucleation of primary recrystallization. Due to the difference in dislocation density, Goss grains are generated in large numbers in the shear bands, where nucleation of Goss grains is likely to occur. From this, it is believed that the number of Goss grains, which increased in number in the primary recrystallization structure, grew during secondary recrystallization in the final annealing, resulting in a refinement of the grain size of the grain-oriented electrical steel sheet and a reduction in iron loss.

[0048] Furthermore, when a steel sheet that has been subjected to aging treatment and holding treatment is subjected to cold rolling or final cold rolling, the steel sheet is less likely to break during rolling. This is thought to be because, by applying the above-mentioned aging treatment conditions during the period from the start of aging treatment after the completion of hot-rolled sheet annealing or final intermediate annealing to the start of cold rolling or final cold rolling, breakage caused by deformation during rolling bite due to the low temperature of the hot-rolled sheet is suppressed.

[0049] In a preferred embodiment, during the temperature rise process when the cold-rolled sheet is subjected to decarburization annealing, the temperature rise rate of the cold-rolled sheet during the temperature rise process excluding the holding treatment is 40°C / s or higher. The temperature rise rate of the cold-rolled sheet during the temperature rise process excluding the holding treatment is more preferably 50°C / s or higher, and even more preferably 70°C / s or higher. By subjecting the cold-rolled sheet to a holding treatment at a temperature rise / decrease rate of -5°C / s or higher and 25°C / s or lower, and raising the temperature during the temperature rise process excluding the holding treatment at a temperature rise rate of 40°C / s or higher, secondary recrystallization of Goss grains is further promoted, and the generation of crystal grains having other orientations is suppressed. The temperature rise process excluding the holding treatment in decarburization annealing includes the temperature rise process before the holding treatment and the temperature rise process after the holding treatment up to the final decarburization annealing temperature.

[0050] When the steel sheet that has undergone decarburization annealing is subsequently designed to emphasize core loss characteristics and form a forsterite film, an annealing separator mainly composed of MgO is applied to the steel sheet surface, dried, and then finish annealed to develop a secondary recrystallized structure highly concentrated in the Goss orientation and form a forsterite film. On the other hand, when the punching workability is emphasized and the formation of a forsterite film is not desired, it is preferable to either not apply an annealing separator or to perform finish annealing using an annealing separator mainly composed of silica, alumina, or the like. When applying the annealing separator, electrostatic application that does not introduce moisture is also effective if a forsterite film is not to be formed. Alternatively, a heat-resistant inorganic material sheet (silica, alumina, mica) may be used instead of the annealing separator.

[0051] When a forsterite film is formed, the annealing temperature for the final annealing is preferably 800°C or higher to induce secondary recrystallization, and the temperature is preferably maintained at 800°C or higher for 15 hours or longer to complete the secondary recrystallization. On the other hand, when a forsterite film is not formed, the annealing temperature is preferably in the range of 850°C or higher and 950°C or lower, since it is sufficient to complete the secondary recrystallization. It is also possible to complete the final annealing by simply maintaining the temperature in this temperature range for several hours or longer. Note that when a purification treatment is performed to emphasize iron loss characteristics or when a forsterite film is formed to reduce transformer noise, it is preferable to raise the temperature to about 1200°C.

[0052] After the final annealing, the steel sheet is then washed with water, brushed, pickled, or the like to remove unreacted annealing separator adhering to the steel sheet surface, followed by flattening annealing to straighten the shape. This is effective in reducing iron loss. This is because the final annealing is generally performed in a coiled state, which can cause the coil to develop a curl, which can degrade the characteristics during iron loss measurement. Furthermore, when steel sheets are used in a stacked configuration, it is effective to form an insulating coating on the steel sheet surface before or after the flattening annealing. In particular, to reduce iron loss, it is preferable to apply a tension-applying coating that can impart tension to the steel sheet as the insulating coating. The tension-applying coating can be formed by applying a tension coating via a binder or by depositing an inorganic substance on the steel sheet surface by physical vapor deposition or chemical vapor deposition, which are preferred methods for forming the tension-applying coating, as these methods provide excellent coating adhesion and a significant iron loss reduction effect.

[0053] To further reduce iron loss, it is preferable to carry out a magnetic domain refining treatment. Examples of the treatment method include, as commonly practiced methods, forming grooves in the final product sheet, introducing linear or point-shaped thermal strain or impact strain by laser irradiation, electron beam irradiation, or plasma irradiation, and etching the surface of an intermediate steel sheet, such as a steel sheet cold-rolled to the final thickness, to form grooves.

[0054] Example 1 A slab containing, by mass%, C: 0.062%, Si: 3.42%, Mn: 0.06%, Al: 0.020%, Se: 0.012%, N: 0.009%, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to a temperature of 1380 ° C, and then hot-rolled to a hot-rolled sheet with a thickness of 2.6 mm. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing at 1010 ° C for 50 seconds, and then to a 1.8 mm intermediate thickness by primary cold rolling, and then to intermediate annealing at 1050 ° C for 30 seconds. The obtained hot-rolled sheet after intermediate annealing immediately before final cold rolling was coiled into a coil, and the coil was placed in a heat-retaining furnace preheated to a predetermined temperature and subjected to aging treatment. The temperature and time of the aging treatment in the heat-retaining furnace at this time were changed as shown in Table 1. Thereafter, final cold rolling was carried out to produce a cold-rolled sheet having a final thickness of 0.23 mm.

[0055] Next, decarburization annealing was performed for 110 seconds at 860 ° C. in a mixed atmosphere of hydrogen and nitrogen. During the temperature rise process of decarburization annealing, the temperature was first raised at a rate of 100 ° C. / s using an induction heating furnace, and during the temperature rise, cooling was performed by heating using a radiant heating furnace or by atmospheric gas injection. The holding temperature T, holding time t, and temperature rise / fall rate shown in Table 1 were achieved by controlling the temperature of the cold-rolled sheet. Next, for samples whose holding temperature during the temperature rise process of decarburization annealing was less than 720 ° C., the temperature was raised to 720 ° C. at a rate of 150 ° C. / s using an induction heating furnace, and then all samples were heated to 860 ° C. at 45 ° C. / s using a radiant heating furnace. Next, an annealing separator mainly composed of MgO was applied to the steel sheet surface, dried, and then finish annealed for 13 hours in a hydrogen atmosphere at a maximum temperature of 1240 ° C. The grain-oriented electrical steel sheet after the finish annealing obtained as described above was subjected to iron loss W measurement according to the method described in JIS C 2556. １７／５０ The results are shown in Table 1.

[0056]

[0057] Table 1 shows that by carrying out the method for producing a grain-oriented electrical steel sheet according to the present invention, which includes intermediate annealing, a grain-oriented electrical steel sheet with low iron loss can be obtained.

[0058] Example 2 A slab containing, by mass%, 0.064% C, 3.32% Si, 0.08% Mn, 0.021% Al, 0.015% Se, and 0.007% N, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to a temperature of 1390 ° C, and then hot-rolled to a hot-rolled sheet having a thickness of 2.2 mm. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing at 1040 ° C for 50 seconds, and the hot-rolled sheet after the hot-rolled sheet annealing was completed was coiled into a coil, and the coil was placed in a heat-retaining furnace preheated to a predetermined temperature and subjected to aging treatment. The temperature and time of the aging treatment in the heat-retaining furnace at this time were changed as shown in Table 2. Then, the cold-rolled sheet was finished into a cold-rolled sheet having a final thickness of 0.23 mm.

[0059] Next, decarburization annealing was performed at 850°C for 120 seconds in a mixed atmosphere of hydrogen and nitrogen. During the temperature rise process of decarburization annealing, the temperature was first raised at a rate of 80°C / s using an induction heating furnace, and during the temperature rise, cooling was performed by heating using a radiant heating furnace or by atmospheric gas injection. The cold-rolled sheet was subjected to temperature control so that the holding temperature T, holding time t, and temperature rise / fall rate were as shown in Table 2, thereby performing a holding treatment. Next, for samples whose holding temperature during the temperature rise process of decarburization annealing was less than 720°C, the temperature was raised to 720°C at a rate of 150°C / s using an induction heating furnace, and then all samples were heated to 850°C at a rate of 50°C / s using a radiant heating furnace. Next, an annealing separator mainly composed of MgO was applied to the steel sheet surface, dried, and then finish annealed for 13 hours in a hydrogen atmosphere at a maximum temperature of 1250°C. For the grain-oriented electrical steel sheets obtained after finish annealing as described above, the iron loss W was measured in the same manner as in Example 1. １７／５０ The results are shown in Table 2.

[0060]

[0061] Table 2 shows that a grain-oriented electrical steel sheet with low iron loss can be obtained by carrying out the method for producing a grain-oriented electrical steel sheet according to the present invention, which does not include intermediate annealing.

[0062] Example 3 A steel slab containing the chemical composition shown in Table 3, with the remainder being Fe and unavoidable impurities, was produced by continuous casting, heated to a temperature of 1380°C, and then hot-rolled to form a hot-rolled sheet with a thickness of 2.3 mm. The resulting hot-rolled sheet was subjected to hot-rolled sheet annealing at 1030°C for 50 seconds, and the hot-rolled sheet after completion of the hot-rolled sheet annealing was then coiled into a coil, which was then loaded into a heat-retaining furnace preheated to 50°C and subjected to aging treatment for 50 hours. The cold-rolled sheet was then finished into a cold-rolled sheet with a final thickness of 0.23 mm.

[0063] Next, decarburization annealing was performed at 800°C for 180 seconds in a mixed atmosphere of hydrogen and nitrogen, followed by nitriding treatment in a mixed atmosphere of hydrogen, nitrogen, and ammonia so that the nitrogen concentration of the steel sheet was 300 ppm. During the temperature rise process of the decarburization annealing, the temperature was first raised at 100°C / s using an induction heating furnace, and during the temperature rise, a holding treatment was performed using a radiant heating furnace at a holding temperature of 500°C, a holding time of 3.0 seconds, and a temperature rise / decrease rate of 5°C / s. Next, the temperature was raised to 720°C at a rate of 150°C / s using an induction heating furnace, and then raised to 800°C at 50°C / s using a radiant heating furnace. Next, an annealing separator mainly composed of MgO was applied to the steel sheet surface, dried, and then finish annealed for 13 hours in a hydrogen atmosphere at 1200°C. The grain-oriented electrical steel sheet obtained after finish annealing as described above was measured for iron loss W in the same manner as in Example 1. １７／５０ The results are shown in Table 3.

[0064]

[0065] Table 3 shows that a grain-oriented electrical steel sheet with low iron loss can also be obtained by using a steel material having a component composition with respect to optional elements as specified in the present invention as a starting material and carrying out the method for producing a grain-oriented electrical steel sheet according to the present invention.

Claims

1. A method for producing a grain-oriented electrical steel sheet, comprising a series of steps: hot rolling a steel material having a chemical composition containing, by mass, 0.002 to 0.100% C, 2.0 to 4.5% Si, 0.005 to 0.50% Mn, with the balance being Fe and unavoidable impurities, to form a hot-rolled sheet; hot-rolling the hot-rolled sheet; then cold-rolling the hot-rolled sheet once or at least twice with intermediate annealing in between 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 finish-annealing the decarburization-annealed sheet, a holding time t (s) of the cold-rolled sheet during the decarburization annealing step, and a holding temperature T (°C) of the cold-rolled sheet during the temperature rise step, wherein the cold-rolled sheet is subjected to a holding treatment in which the temperature rise / fall rate of the cold-rolled sheet is set to be -5°C / s or more and 25°C / s or less, and wherein the holding time t and the holding temperature T satisfy all of the following formulas (1) to (3): T≦1500t-800 (1) 400≦T≦700 (2) t≦8.0 (3) 2. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein the temperature rise rate of the cold-rolled sheet during the temperature rise process excluding the holding treatment during the decarburization annealing is 40°C / s or more.

3. The chemical composition is: Group A: in mass %, one or more selected from Al: 0.005 to 0.050%, N: 0.003 to 0.020%, S: 0.002 to 0.030%, Se: 0.003 to 0.030%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, 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: one or more elements selected from, 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: one or more elements selected from, in 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%; 3. The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, further comprising at least one group of one or more elements selected from Group D, in 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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