Manufacturing method of grain-oriented electrical steel sheets

Abstract

The present invention provides a method for producing grain-oriented electrical steel sheets with low iron loss that can prevent fracture of the steel sheet during final cold rolling and refine the goss grains after secondary recrystallization annealing. A steel sheet having a predetermined component composition is subjected to hot-rolled sheet annealing or final intermediate annealing, and then 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 higher and 150 hours or lower. After that, it is subjected to final cold rolling to produce a cold-rolled sheet, and during the heating process when decarburizing annealing is performed on the cold-rolled sheet, a holding treatment is performed in which the heating rate of the cold-rolled sheet is set to -5°C / s or higher and 25°C / s or lower, so that the holding time t(s) and holding temperature T(°C) satisfy all of the following equations: T≦1500t-800, 400≦T≦700, and t≦8.0.

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing grain-oriented electrical steel sheets. [Background technology]

[0002] Grain-oriented electrical steel sheets are soft magnetic material primarily used as core material for transformers. Grain-oriented electrical steel sheets are iron with an easy magnetization axis. <001> The steel sheet has a crystalline structure in which the orientation is highly aligned with the rolling direction of the steel sheet, and the finer the crystal grains, the lower the iron loss. As a method to reduce iron loss by refining the crystal grains of grain-oriented electrical steel sheets, for example, Patent Document 1 describes that it is effective to perform a holding treatment in the middle of the heating process during decarburization annealing, which also serves as primary recrystallization annealing, in which the temperature of the steel sheet is maintained at a predetermined temperature for a certain period of time. According to this method, by performing a holding treatment during decarburization annealing, the temperature inside the steel sheet is made uniform, <111> / / ND orientation preferentially recovers after primary recrystallization <111> / / It is believed that a decrease in the ND orientation and an increase in Goss nuclei, resulting in finer recrystallized grains after secondary recrystallization, leads to the production of low iron loss grain-oriented electrical steel sheets. Here, ND refers to the direction of the normal to the surface of the steel sheet. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2014 / 017589 [Overview of the project] [Problems that the invention aims to solve]

[0004] In the general manufacturing process of grain-oriented electrical steel sheets, {110} is selected from the primary recrystallized grains. <001> Goss grains with a specific orientation undergo significant grain growth through secondary recrystallization. This coarsening of secondary recrystallized grains leads to an increase in eddy current losses, which in turn increases iron losses. Furthermore, the coarsening of secondary recrystallized grains presents a problem where the steel sheet is more prone to fracture during the final cold rolling process, and a solution has been sought.

[0005] Therefore, in view of the above problems, the present invention aims to provide a method for manufacturing grain-oriented electrical steel sheets that can prevent fracture of the steel sheet during final cold rolling and produce grain-oriented electrical steel sheets with low iron loss and finer goss grains after secondary recrystallization annealing. [Means for solving the problem]

[0006] The inventors conducted extensive research to solve the above problems. As a result, they discovered that by applying an aging treatment to a steel sheet that has been annealed immediately before the final cold rolling, followed by the final cold rolling, and then applying a holding treatment to the cold-rolled sheet after the final cold rolling under specific conditions, the secondary recrystallized grains become finer, and the iron loss of grain-oriented electrical steel sheets is further reduced compared to conventional technology, thus completing the present invention.

[0007] The gist of the present invention is as follows:

[0008] [1] A steel material having a composition by mass %, containing C: 0.002~0.100%, Si: 2.0~4.5%, Mn: 0.005~0.50%, with the remainder being Fe and unavoidable impurities, is subjected to hot rolling to form a hot-rolled sheet. The hot-rolled sheet is subjected to hot-rolled sheet annealing. Subsequently, the hot-rolled sheet is subjected to cold rolling once or two or more times with an intermediate annealing in between to obtain a cold-rolled sheet with a final thickness. The aforementioned cold-rolled sheet is subjected to decarburization annealing, which also serves as primary recrystallization annealing, to obtain a decarburized annealed sheet. A method for manufacturing grain-oriented electrical steel sheets, comprising a series of steps including applying an annealing release agent to the surface of the decarburized annealed sheet and performing finish annealing on the decarburized annealed sheet, (I) When cold rolling is performed once, after hot-rolled sheet annealing and before cold rolling, (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, an 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. A method for manufacturing grain-oriented electrical steel sheets, characterized in that, during the heating process when performing the decarburization annealing on the cold-rolled sheet, a holding treatment is performed such that the heating rate of the cold-rolled sheet is -5°C / s or more and 25°C / s or less, 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), and 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 manufacturing a grain-oriented electrical steel sheet according to [1], wherein the heating rate of the cold-rolled sheet in the heating process excluding the holding treatment when subjecting the cold-rolled sheet to the decarburization annealing is 40°C / s or more.

[0010] [3] The component composition is Group A: One or more elements selected by mass%, from Al: 0.005~0.050%, N: 0.003~0.020%, S: 0.002~0.030%, Se: 0.003~0.030%, Sn: 0.005~0.500%, Cr: 0.005~0.500%, Cu: 0.01~0.50%, Ni: 0.01~0.50%, Bi: 0.005~0.500%, P: 0.005~0.500%, Sb: 0.005~0.500%, and Mo: 0.005~0.500%. Group B: One or more elements selected from the following in mass percent or mass ppm: B: 0.1~25.0 ppm, Nb: 0.001~0.020%, Ti: 0.0005~0.0400%, V: 0.001~0.020%, and Co: 0.002~0.050%. Group C: In mass%, one or more selected from As: 0.0010~0.0200%, Pb: 0.0001~0.0100%, W: 0.0010~0.0100%, and Zn: 0.001~0.020%, and Group D: One or more selected from the group consisting of, 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% The method for manufacturing a grain-oriented electrical steel sheet according to [1] or [2] above, further comprising at least one group of .

Advantages of the Invention

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

Brief Description of the Drawings

[0012] [Figure 1] It is a graph showing the relationship between the aging treatment time and the iron loss. [Figure 2] It is a graph showing the relationship between the aging treatment temperature and the iron loss. [Figure 3] It is a graph showing the relationship between the heating and cooling rates of the holding treatment in the temperature rising process of decarburization annealing and the iron loss. [Figure 4] It is a graph showing the range of the holding time t and the holding temperature T in the holding treatment.

Embodiments for Carrying Out the Invention

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

[0014] <Experiment 1> A steel material containing, by mass%, C: 0.068%, Si: 3.31%, Mn: 0.07%, Se: 0.016%, S: 0.006%, and the balance being Fe and inevitable impurities was produced by a continuous casting method. The obtained steel material was heated to a temperature of 1380 °C and then hot-rolled to obtain a hot-rolled sheet with a thickness of 2.5 mm. After subjecting the obtained hot-rolled sheet to hot-rolled sheet annealing at 1010 °C for 60 s (seconds), primary cold rolling was performed to obtain an intermediate plate thickness of 1.7 mm, and intermediate annealing was performed at 1050 °C for 25 s to obtain a plurality of steel sheets. The obtained steel sheets were charged into a holding furnace whose temperature was controlled at 70 °C, and after performing aging treatment under 19 time conditions from 10 h (hours) to 250 h, final cold rolling was performed to finish into a cold-rolled sheet with a final thickness of 0.23 mm. Next, the obtained cold-rolled sheet was subjected to decarburization annealing that also served as primary recrystallization annealing in a mixed gas atmosphere of hydrogen and nitrogen. In the heating-up process of the decarburization annealing, first, a radiant heating furnace was used to heat up the cold-rolled sheet under the condition that the heating-up rate was 50 °C / s. Next, during the heating-up process, the output of the radiant heating furnace was adjusted to make the heating and cooling rates of the cold-rolled sheet ±0 °C / s, and a holding treatment was performed where the holding temperature of the cold-rolled sheet was 540 °C and the holding time was 3.0 s. Next, an induction heating furnace was used to heat up the cold-rolled sheet at a heating-up rate of 150 °C / s to 720 °C, and then, again, a radiant heating furnace was used to heat up at a heating-up rate of 40 °C / s to 830 °C, and decarburization annealing was performed at 830 °C for 100 s. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet, and after drying the annealing separating agent, finish annealing was performed in a hydrogen atmosphere at a maximum temperature of 1220 °C for 10 h to obtain 19 types of grain-oriented electrical steel sheets with different aging treatment times. For the obtained grain-oriented electrical steel sheets, according to the method described in Japanese Industrial Standard JIS C 2556, the iron loss W 17 / 50 was measured. The results are shown in Fig. 1. According to Fig. 1, it can be seen that when the temperature of the aging treatment is 70 °C, a grain-oriented electrical steel sheet with low iron loss can be obtained when the aging treatment time is 24 h or more and 150 h or less.

[0015] <Experiment 2> The steel sheet that underwent intermediate annealing in Experiment 1 was charged into a heat-retaining furnace controlled to 19 different temperatures ranging from 5°C to 250°C. After 100 hours of aging treatment, it was subjected to final cold rolling 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, under a mixed gas atmosphere of hydrogen and nitrogen. In the heating process of decarburization annealing, the cold-rolled sheet was first heated using an induction heating furnace at a heating rate of 150°C / s. Then, midway through the heating process, the cold-rolled sheet was heated using a radiant heating furnace at a heating / cooling rate of 10°C / s, and held at a temperature of 600°C for 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 again to 840°C at a heating rate of 45°C / s using a radiant heating furnace, followed by decarburization annealing at 840°C for 110 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, finish annealing was performed in a hydrogen atmosphere at a maximum temperature of 1200°C for 12 hours to obtain 19 types of grain-oriented electrical steel sheets with different aging treatment temperatures. Iron loss W was measured for the obtained grain-oriented electrical steel sheets using the same method as in Experiment 1. 17 / 50 The following measurements were taken. The results are shown in Figure 2. According to Figure 2, when the aging treatment time is 100 hours, a grain-oriented electrical steel sheet with low iron loss can be obtained when the aging treatment temperature is between 30°C and 150°C.

[0016] <Experiment 3> The steel sheet that underwent intermediate annealing in Experiment 1 was charged into a heat-retaining furnace controlled to a temperature of 50°C, subjected to 100 hours of aging treatment, 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 under a mixed gas atmosphere of hydrogen and nitrogen. In the heating process of decarburization annealing, the cold-rolled sheet was first heated using an induction heating furnace at a heating rate of 70°C / s. Then, during the heating process, the heating and cooling rates of the cold-rolled sheet were varied to 19 different conditions ranging from -10°C / s to 30°C / s using a radiant heating furnace and atmospheric gas injection, and the cold-rolled sheet was held at a holding temperature of 530°C for a holding time of 5.0 s. Here, the holding temperature of 530°C is the midpoint between the temperature of the cold-rolled sheet at the beginning and the temperature at the end of the 5.0 s holding time. 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 again to 810°C at a heating rate of 50°C / s using a radiant heating furnace, followed by decarburization annealing 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 drying the annealing separator, finish annealing was performed in a hydrogen atmosphere at a maximum temperature of 1220°C for 11 hours to obtain 19 types of grain-oriented electrical steel sheets with different heating and cooling rates for the holding treatment. Iron loss W was measured for the obtained grain-oriented electrical steel sheets using the same method as in Experiment 1. 17 / 50 The following measurements were taken. The results are shown in Figure 3. According to Figure 3, when the holding temperature of the holding process is 530°C and the holding time is 5.0 s, it can be seen that a grain-oriented electrical steel sheet with low iron loss can be obtained when the heating and cooling rate of the holding process is between -5°C / s and 25°C / s.

[0017] <Experiment 4> The steel sheet that underwent intermediate annealing in Experiment 1 was charged into a heat-retaining furnace controlled to a temperature of 80°C, subjected to 50 hours of aging treatment, 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 under a mixed gas atmosphere of hydrogen and nitrogen. In the heating process of decarburization annealing, the cold-rolled sheet was first heated using an induction heating furnace at a heating rate of 250°C / s. Then, midway through the heating process, the cold-rolled sheet was heated using a radiant heating furnace at a heating / cooling rate of 15°C / s, and the sheet was held in 26 different combinations, with holding temperatures ranging from 380°C to 720°C and holding times ranging from 0.6 s to 10.0 s. Here, the holding temperature is the temperature midway between the temperature of the cold-rolled sheet at the beginning and the temperature at the end of the holding time. Next, for samples where the holding temperature during the holding treatment was less than 720°C, the temperature was raised to 720°C using an induction heating furnace at a heating rate of 150°C / s. Then, for all samples, the temperature was raised to 870°C using a radiant heating furnace at a heating rate of 40°C / s, and decarburization annealing was performed at 870°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, finish annealing was performed in a hydrogen atmosphere at a maximum temperature of 1230°C for 10 hours to obtain 26 types of grain-oriented electrical steel sheets with different holding temperatures and holding times during the holding treatment. Iron loss W was measured for the obtained grain-oriented electrical steel sheets using the same method as in Experiment 1. 17 / 50 The iron loss W was measured. The results are shown in Figure 4. In Figure 4, the circle indicates the measured iron loss W. 17 / 50 This indicates that the value is 0.830 W / kg or less, and the ● mark indicates iron loss in W. 17 / 50 This indicates that the iron loss is greater than 0.830 W / kg. As shown in Figure 4, when the heating and cooling rate of the holding process is 15°C / s, and the holding time is t (s) and the holding temperature is T (°C), it can be seen that low iron loss grain-oriented electrical steel sheets can be obtained within the specified range that satisfies all of the following equations (1) to (3). T ≤ 1500t - 800 (1) 400 ≤ T ≤ 700 (2) t≦8.0 (3)

[0018] Next, embodiments for carrying out the present invention will be described.

[0019] In one embodiment, the present invention is A method for manufacturing grain-oriented electrical steel sheets, comprising a series of steps including: hot rolling a steel material having a composition by mass% containing C: 0.002~0.100%, Si: 2.0~4.5%, Mn: 0.005~0.50%, with the remainder being Fe and unavoidable impurities to obtain a hot-rolled sheet; hot-rolling the hot-rolled sheet to obtain a hot-rolled sheet; then cold-rolling the hot-rolled sheet once or two or more times with an intermediate annealing in between to obtain a cold-rolled sheet having a final thickness; decarburizing the cold-rolled sheet while performing primary recrystallization annealing to obtain a decarburized annealed sheet; applying an annealing release agent to the surface of the decarburized annealed sheet; and finishing the decarburized annealed sheet, the method comprising these steps. (I) When cold rolling is performed once, after hot-rolled sheet annealing and before cold rolling, (II) When cold rolling is performed two or more times, after the final intermediate annealing performed immediately before the final cold rolling and before the final cold rolling, an 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. The invention relates to a method for manufacturing grain-oriented electrical steel sheets, characterized in that, during the heating process when performing the decarburization annealing on a cold-rolled sheet, a holding treatment is performed such that the heating rate of the cold-rolled sheet is between -5°C / s and 25°C / s, the time from the start to the end of the holding treatment is defined as the holding time t(s), and the average temperature of the cold-rolled sheet during the holding treatment is defined as the holding temperature T(°C), and the holding time t and holding temperature T satisfy all of the following formulas (1) to (3). T ≤ 1500t - 800 (1) 400 ≤ T ≤ 700 (2) t≦8.0 (3)

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

[0021] C: 0.002~0.100% If the carbon content (C) is less than 0.002%, the grain boundary strengthening effect of C is lost, leading to defects that hinder manufacturing, such as cracks in the slab. On the other hand, if it exceeds 0.100%, it becomes difficult to reduce it to 0.005% or less, where magnetic aging does not occur, through decarburization annealing. Therefore, C should be in the range of 0.002% to 0.100%. Preferably, C should be in the range of 0.010% to 0.080%.

[0022] Si: 2.0~4.5% Si is an element necessary to increase the resistivity of steel and reduce iron loss. The above effect is not sufficient if the Si content is less than 2.0%, while if it exceeds 4.5%, the workability decreases, making it difficult to manufacture by rolling. Therefore, the Si content should be in the range of 2.0 to 4.5%. Preferably, the S content should be in the range of 2.5 to 4.0%.

[0023] Mn: 0.005~0.50% Mn is an element necessary to improve the hot workability of steel. The above effect is not sufficient if the amount is less than 0.005%, while if it exceeds 0.50%, the magnetic flux density of the product sheet decreases. Therefore, Mn should be in the range of 0.005 to 0.50%. Preferably, Mn should be in the range of 0.02 to 0.20%. The above C, Si, and Mn are essential elements contained in the steel material used in the manufacture of grain-oriented electrical steel sheets according to the present invention.

[0024] Furthermore, the steel material has a component composition consisting of Fe and unavoidable impurities as the remainder. Here, unavoidable impurities refer to impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, etc., and whose inclusion is permissible to the extent that it does not hinder the purpose of the present invention. Examples of raw materials containing unavoidable impurities include iron ore, reduced iron, or scrap.

[0025] In other embodiments, the component composition is Group A: One or more elements selected by mass%, from Al: 0.005~0.050%, N: 0.003~0.020%, S: 0.002~0.030%, Se: 0.003~0.030%, Sn: 0.005~0.500%, Cr: 0.005~0.500%, Cu: 0.01~0.50%, Ni: 0.01~0.50%, Bi: 0.005~0.500%, P: 0.005~0.500%, Sb: 0.005~0.500%, and Mo: 0.005~0.500%. Group B: One or more elements selected from the following in mass percent or mass ppm: B: 0.1~25.0 ppm, Nb: 0.001~0.020%, Ti: 0.0005~0.0400%, V: 0.001~0.020%, and Co: 0.002~0.050%. Group C: In mass%, one or more selected from As: 0.0010~0.0200%, Pb: 0.0001~0.0100%, W: 0.0010~0.0100%, and Zn: 0.001~0.020%, and Group D: One or more elements selected by mass%, from Ag: 0.001-0.050%, Au: 0.001-0.050%, Ca: 0.001-0.020%, Ga: 0.0001-0.0050%, Ge: 0.0001-0.0050%, Nd: 0.001-0.020%, and La: 0.001-0.020%. It further includes at least one group of these elements. These elements are optional elements present in the steel material in addition to the essential elements mentioned above.

[0026] The above-mentioned arbitrary elements will be explained separately for cases where an inhibitor is used to induce secondary recrystallization and cases where it is not. First, when an inhibitor is used to induce secondary recrystallization, it is preferable to include Al and N in the ranges of Al: 0.005~0.050% and N: 0.003~0.020%, respectively. When using a MnS·MnSe-based inhibitor, it is preferable to include the aforementioned amount of Mn and one or two of S: 0.002~0.030% and Se: 0.003~0.030%. If the amount added is less than the lower limit above, the inhibitor effect will not be sufficiently obtained. On the other hand, if the amount added exceeds the upper limit above, the inhibitor component will remain undissolved when the slab is heated, leading to a decrease in magnetic properties. Note that AlN-based and MnS·MnSe-based inhibitors may be used simultaneously.

[0027] On the other hand, when inhibitors are not used to induce secondary recrystallization, it is preferable to reduce the content of Al, N, S, and Se, which are the inhibitor-forming components mentioned above, as much as possible. Specifically, it is preferable to reduce the content of these elements in the steel material to less than 0.005% for Al, less than 0.0050% for N, less than 0.0050% for S, and less than 0.0030% for Se.

[0028] However, regardless of whether or not inhibitors are used, for the purpose of improving magnetic properties, the steel material may further contain one or more arbitrary elements selected from Sn: 0.005-0.500%, Cr: 0.005-0.500%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Bi: 0.005-0.500%, P: 0.005-0.500%, Sb: 0.005-0.500%, and Mo: 0.005-0.500%. All of the arbitrary elements described above belong to Group A.

[0029] Furthermore, also for the purpose of improving magnetic properties, the steel material is given one or more elements selected from the following: Group B: B: 0.1~25.0 ppm, Nb: 0.001~0.020%, Ti: 0.0005~0.0400%, V: 0.001~0.020%, and Co: 0.002~0.050%, and Group C: As: 0.0010~0.0200%, Pb: 0.0001~0.0100%, W: 0.0010~0.0100%. In other embodiments, the composition of the steel material further includes, in addition to the essential elements, at least one of the above groups A to D. Any combination of groups A to D is acceptable.

[0030] 2. Manufacturing method Next, the method for manufacturing grain-oriented electrical steel sheets according to the present invention will be described. After melting steel having the above-mentioned component composition using a conventional refining process, slabs obtained by the conventionally known ingot-parting rolling method or continuous casting method can be used as the steel material of the present invention. Alternatively, thin cast slabs with a thickness of 100 mm or less obtained by the direct casting method may be used as the steel material. The steel material is prepared according to conventional methods; for example, if it contains inhibitor components, it is heated to about 1400°C, while if it does not contain inhibitor components, it is heated to a temperature of 1250°C or lower, and then hot-rolled to produce a hot-rolled sheet. If it does not contain inhibitor components, 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 to obtain good magnetic properties. Below 800°C, the band structure formed by hot rolling remains, making it difficult to obtain a uniform primary recrystallized structure, and inhibiting the development of secondary recrystallization. On the other hand, above 1150°C, the grain size after hot-rolled sheet annealing becomes too coarse, again making it difficult to obtain a uniform primary recrystallized structure.

[0032] Next, the hot-rolled sheet, after annealing, is subjected to one cold rolling or two or more cold rollings with an intermediate annealing in between, to obtain a cold-rolled sheet of the final thickness. The annealing temperature for the intermediate annealing is preferably in the range of 900 to 1200°C. Below 900°C, the recrystallized grains after intermediate annealing become finer, and furthermore, the number of Goss nuclei in the primary recrystallized structure decreases, which tends to reduce the magnetic properties of the grain-oriented electrical steel sheet. On the other hand, above 1200°C, as with the hot-rolled sheet annealing, the crystal grains become too coarse, making it difficult to obtain a primary recrystallized structure with uniform grains.

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

[0034] It is important to subject the steel sheet to an aging treatment between the completion of hot-rolled sheet annealing or final intermediate annealing and the commencement of cold rolling or final cold rolling, maintaining a temperature of 30°C or higher and 150°C or lower for 24 hours or more and 150 hours or less. In this case, the start of the aging treatment period shall be when the temperature of the steel sheet reaches 30°C or higher and 150°C or lower. The end of the aging treatment period shall be 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 commenced while the temperature of the steel sheet is 30°C or higher and 150°C or lower, the end of the aging treatment period shall be when final cold rolling commences. The temperature of the steel sheet can be monitored by known means.

[0035] If the aging treatment temperature is below 30°C, the temperature of the steel sheet at the time of jamming during the final cold rolling is low, which can cause fracture. Also, carbon and nitrogen do not diffuse and instead segregate, making it difficult for uniform shear bands to form in the steel during the final cold rolling. On the other hand, if the aging treatment temperature exceeds 150°C, the mobility of carbon and nitrogen increases, which conversely leads to segregation at the grain boundaries, making it difficult for uniform shear bands to form in the steel during cold rolling. If the aging treatment time is less than 24 hours, it is too short for carbon and nitrogen to diffuse and disperse in the steel, and if it exceeds 150 hours, the diffusion time is too long, which tends to lead to grain boundary segregation.

[0036] The aging treatment of the hot-rolled sheet described above can be carried out inside the annealing furnace immediately after the completion of hot-rolled sheet annealing or final intermediate annealing.

[0037] If it is difficult to meet the above-mentioned temperature and time conditions for aging treatment inside the annealing furnace, the steel sheet that has undergone hot-rolled sheet annealing or final intermediate annealing may be wound into a coil, and the coil may be kept warm in a temperature-controlled heating furnace to perform aging treatment on the steel sheet. In this case, the start of the aging treatment time shall 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 shall 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 shall be the moment the coil is discharged when final cold rolling is started. The surface temperature of the coil can be monitored by known means.

[0038] In the final cold rolling process, applying a thermal treatment to the steel sheet, heating it to a temperature between 100°C and 300°C, once or multiple times during the cold rolling process, is effective in improving the primary recrystallized texture and enhancing 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 is preferably in the range of 800°C to 900°C. Furthermore, the atmosphere for decarburization annealing is preferably a humid atmosphere under a mixed gas atmosphere of hydrogen and nitrogen, as 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 preferable. However, this does not apply if the steel material contains less than 0.005% carbon and decarburization is unnecessary. Note that primary recrystallization annealing may be performed separately from decarburization annealing.

[0040] In decarburization annealing, the important thing is that, during the heating process when decarburizing a cold-rolled sheet, a holding treatment is performed where the heating rate of the cold-rolled sheet is between -5°C / s and 25°C / s, and the holding 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), and the holding time t and holding temperature T satisfy all of the following equations (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 point when the heating rate of the cold-rolled sheet is reduced from the initial high speed to a certain heating / cooling rate of -5°C / s or more and 25°C / s or less, which is set as the heating / cooling rate in the holding process. The end of the holding time t is defined as the point when the heating rate of the cold-rolled sheet is accelerated from a certain heating / cooling rate set as the heating / cooling rate in the holding process back to the initial high speed.

[0042] Furthermore, the holding temperature T is defined as the time-averaged temperature of the cold-rolled sheet during the holding time t, from the beginning to the end. If the temperature of the cold-rolled sheet hardly changes during the holding time t, that temperature may be considered the holding temperature T. The temperature of the cold-rolled sheet can be monitored by known means.

[0043] If the heating and cooling rate of the cold-rolled sheet during the holding process is slower than -5°C / s (if the cooling rate is faster than 5°C / s), iron loss increases. Similarly, if the heating and cooling rate is faster than 25°C / s, iron loss also increases.

[0044] To control the heating and cooling rates of the cold-rolled sheet during the holding process within the above range, for example, this can be done by controlling the output of a radiant or induction heating furnace during heating, and by controlling the injection of atmospheric gas during cooling. Here, the temperature history of the cold-rolled sheet is irrelevant as long as the holding time t and holding temperature T satisfy the above conditions. The temperature history of the cold-rolled sheet may be, for example, one or more instantaneous heating cycles using pulse waves, multi-stage heating, multi-stage cooling, slow heating, slow cooling, and uniform heat holding.

[0045] Although the exact reason for the low iron loss in grain-oriented electrical steel sheets subjected to aging and holding treatments under the above conditions during the manufacturing process is unclear, the inventors believe that it may be due to the refinement of the grain size of the secondary recrystallized grains in the grain-oriented electrical steel sheets through the mechanism described below.

[0046] First, by applying the aforementioned aging treatment conditions, the carbon (C) and nitrogen (N) contained in the steel sheet can be kept dispersed without segregation. Performing the final cold rolling with the C and N dispersed increases the number of shear bands. Since goss grains originate from the shear bands, applying the aforementioned aging treatment to the hot-rolled sheet increases the number of goss grains.

[0047] Next, when performing decarburization annealing combined with primary recrystallization annealing, applying the above conditions causes recovery of the cold-rolled sheet, and most dislocations are eliminated in areas other than the shear zone, but it is thought that some dislocations remain in the shear zone, which is a region where many dislocations exist. As a result, a difference in dislocation density is created between the shear zone and the non-shear zone. This difference in dislocation density becomes the driving force for nucleation in primary recrystallization. Due to the difference in dislocation density, many Goss grains are generated from the shear zone, where Goss grain nucleation is more likely to occur. From this, it is thought that the increased number of Goss grains in the primary recrystallized structure undergo grain growth through secondary recrystallization during finish annealing, resulting in a finer grain size in the grain-oriented electrical steel sheet and a reduction in iron loss.

[0048] Furthermore, when cold-rolling or final cold-rolling steel sheets that have undergone aging and holding treatments are performed, fracture of the steel sheet during rolling is less likely to occur. This is thought to be because applying the aforementioned aging treatment conditions between the start of aging treatment after hot-rolled sheet annealing or final intermediate annealing is completed and the start of cold-rolling or final cold-rolling suppresses fracture caused by deformation during rolling due to the low temperature of the hot-rolled sheet.

[0049] In a preferred embodiment, the heating rate of the cold-rolled sheet during the heating process excluding the holding treatment when decarburizing and annealing the cold-rolled sheet is set to 40°C / s or higher. More preferably, the heating rate of the cold-rolled sheet during the heating process excluding the holding treatment is 50°C / s or higher, and even more preferably 70°C / s or higher. By performing a holding treatment on the cold-rolled sheet with a heating / cooling rate of -5°C / s or higher and 25°C / s or lower, and heating it at a heating rate of 40°C / s or higher during the heating process excluding the holding treatment, secondary recrystallization of Goss grains is further promoted, and the formation of crystal grains with other orientations is suppressed. The heating process excluding the holding treatment in decarburizing and annealing includes the heating process before the holding treatment and the heating process after the holding treatment up to the final decarburizing and annealing temperature.

[0050] When a steel sheet has undergone decarburization annealing and the formation of a forsterite coating is desired, an annealing separator mainly composed of MgO is applied to the surface of the steel sheet, dried, and then a finish annealing is performed to develop a secondary recrystallized structure with high concentration in the Goss orientation, and to form a forsterite coating. On the other hand, when the formation of a forsterite coating is desired and the form of punching workability is desired, it is preferable not to apply an annealing separator, or to perform a finish annealing using an annealing separator mainly composed of silica or alumina. Furthermore, electrostatic coating, which does not introduce moisture, is also effective when a forsterite coating is not desired. Alternatively, a heat-resistant inorganic material sheet (silica, alumina, mica) may be used instead of the annealing separator.

[0051] For finish annealing, if a forsterite film is to be formed, it is preferable to anneal at 800°C or higher to induce secondary recrystallization, and to hold at a temperature of 800°C or higher for 15 hours or more to complete the secondary recrystallization. On the other hand, if a forsterite film is not to be formed, it is sufficient for secondary recrystallization to be completed, so the annealing temperature is preferably in the range of 850°C to 950°C, and it is possible to complete the finish annealing by holding at this temperature range for several hours or more. Furthermore, if purification treatment is performed to emphasize iron loss characteristics, or if a forsterite film is formed to reduce transformer noise, it is preferable to raise the temperature to around 1200°C.

[0052] After finish annealing, it is effective to perform a planar annealing process to correct the shape of the steel sheet by washing, brushing, or pickling to remove any unreacted annealing release agent adhering to the surface, thereby reducing iron loss. This is because finish annealing is generally performed in a coil state, which can cause coil deformation and degrade the characteristics during iron loss measurement. Furthermore, when using laminated steel sheets, it is effective to form an insulating film on the surface of the steel sheet before or after the planar annealing process described above. In particular, to reduce iron loss, it is preferable to apply a tension-imparting film that can impart tension to the steel sheet as the insulating film. When forming the tension-imparting film, it is preferable to use a method of applying the tension film via a binder, or a method of depositing inorganic material onto the surface of the steel sheet using physical vapor deposition or chemical vapor deposition, as these methods can form an insulating film with excellent adhesion and a significantly large iron loss reduction effect.

[0053] Furthermore, in order to further reduce iron loss, it is preferable to perform magnetic domain subdivision processing. As for processing methods, commonly used methods include forming grooves in the final product plate, introducing thermal strain or impact strain in a linear or point-like manner by laser irradiation, electron beam irradiation, or plasma irradiation, or forming grooves by etching the surface of an intermediate steel plate, such as a cold-rolled steel plate to the final plate thickness. [Examples]

[0054] <Example 1> A slab containing, by mass%, C:0.062%, Si:3.42%, Mn:0.06%, Al:0.020%, Se:0.012%, and N:0.009%, with the remainder being Fe and unavoidable impurities, was manufactured by continuous casting. After heating to 1380°C, it was hot-rolled to a hot-rolled sheet with a thickness of 2.6 mm. The obtained hot-rolled sheet was hot-rolled annealed at 1010°C for 50 s, then cold-rolled to an intermediate thickness of 1.8 mm, followed by intermediate annealing at 1050°C for 30 s. The hot-rolled sheet obtained after the intermediate annealing immediately preceding the final cold rolling was wound into a coil, and the coil was charged into a preheated furnace for aging treatment. The temperature and aging time in the furnace were varied as shown in Table 1. Subsequently, a final cold rolling process was performed to produce a cold-rolled sheet with a final thickness of 0.23 mm.

[0055] Next, decarburization annealing was performed at 860°C for 110 s in a mixed atmosphere of hydrogen and nitrogen. During the heating process of decarburization annealing, the temperature of the cold-rolled sheet was controlled by first heating it at a rate of 100°C / s using an induction heating furnace, and then cooling it by heating it using a radiant heating furnace or by injecting atmospheric gas during the heating process, so that the holding temperature T, holding time t, and heating / cooling rate were as shown in Table 1. Next, for samples where the holding temperature during the heating process of decarburization annealing was below 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 a rate of 45°C / s using a radiant heating furnace. Subsequently, an annealing separation agent mainly composed of MgO was applied to the surface of the steel sheet, dried, and then a finish annealing was performed for 13 hours in a hydrogen atmosphere at a maximum temperature of 1240°C. The grain-oriented electrical steel sheet obtained as described above after finish annealing was subjected to iron loss W according to the method described in JIS C 2556. 17 / 50 The following measurements were taken. The results are shown in Table 1.

[0056] [Table 1]

[0057] Table 1 shows that by implementing the manufacturing method for grain-oriented electrical steel sheets according to the present invention, including intermediate annealing, grain-oriented electrical steel sheets with low iron loss can be obtained.

[0058] <Example 2> A slab containing, by mass%, C:0.064%, Si:3.32%, Mn:0.08%, Al:0.021%, Se:0.015%, and N:0.007%, with the remainder being Fe and unavoidable impurities, was manufactured by continuous casting. After heating to 1390°C, it was hot-rolled to a hot-rolled sheet with a thickness of 2.2 mm. The obtained hot-rolled sheet was hot-rolled and annealed at 1040°C for 50 seconds. After the annealing was completed, the hot-rolled sheet was wound into a coil, and the coil was charged into a preheated furnace for aging treatment. The temperature and aging time in the furnace were varied as shown in Table 2. Subsequently, it was cold-rolled to a cold-rolled sheet with a final thickness of 0.23 mm.

[0059] Next, decarburization annealing was performed at 850°C for 120 s in a mixed atmosphere of hydrogen and nitrogen. During the heating process of decarburization annealing, the temperature was initially raised at a rate of 80°C / s using an induction heating furnace, and during the heating process, the temperature of the cold-rolled sheet was controlled to achieve the holding temperature T, holding time t, and heating / cooling rate shown in Table 2. Next, for samples where the holding temperature during the heating process of decarburization annealing was below 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. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet, dried, and then finish annealing was performed for 13 hours in a hydrogen atmosphere at a maximum temperature of 1250°C. Iron loss W of the grain-oriented electrical steel sheet obtained after the finish annealing described above was determined using the same method as in Example 1. 17 / 50 The following measurements were taken. The results are shown in Table 2.

[0060] [Table 2]

[0061] According to Table 2, it can be seen that a non-oriented electrical steel sheet with low iron loss can also be obtained by implementing the method for manufacturing a grain-oriented electrical steel sheet according to the present invention that does not include intermediate annealing.

[0062] <Example 3> A steel slab containing the component composition described in Table 3 and containing Fe and inevitable impurities as the balance was manufactured by a continuous casting method. After heating to a temperature of 1380°C, hot rolling was performed to obtain a hot-rolled sheet with a thickness of 2.3 mm. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing at 1030°C for 50 s. After the hot-rolled sheet annealing was completed, the hot-rolled sheet was wound up to form a coil, and the coil was charged into a holding furnace preheated to 50°C and subjected to aging treatment for 50 h. Thereafter, cold rolling was performed to finish into a cold-rolled sheet with a final thickness of 0.23 mm.

[0063] Next, after decarburization annealing at 800°C for 180 s in a mixed atmosphere of hydrogen and nitrogen, nitriding treatment was performed to make the nitrogen concentration of the steel sheet 300 ppm in a mixed atmosphere of hydrogen, nitrogen and ammonia. In the temperature rising process of the decarburization annealing, first, the temperature was raised at 100°C / s using an induction heating furnace. During the temperature rising process, a holding treatment was performed using a radiation heating furnace at a holding temperature of 500°C and a holding time of 3.0 s to make the temperature rising and falling rate 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 the temperature was raised to 800°C at 50°C / s using a radiation heating furnace. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet and dried, and then final annealing was performed in a hydrogen atmosphere at 1200°C for 13 h. Regarding the grain-oriented electrical steel sheet after final annealing obtained as described above, the iron loss W 17 / 50 was measured in the same manner as in Example 1. The results are shown in Table 3.

[0064]

Table 3

[0065] According to Table 3, it can be seen that a grain-oriented electrical steel sheet with low iron loss can also be obtained by using a steel material having the component composition defined in the present invention for any element as a starting material and implementing the method for manufacturing a grain-oriented electrical steel sheet according to the present invention.

Claims

1. A steel material having a composition in mass percent of C: 0.002 to 0.100%, Si: 2.0 to 4.5%, Mn: 0.005 to 0.50%, with the remainder being Fe and unavoidable impurities, is subjected to hot rolling to produce a hot-rolled sheet. The hot-rolled sheet is subjected to hot-rolled sheet annealing. Subsequently, the hot-rolled sheet is subjected to cold rolling once or two or more times with an intermediate annealing in between to obtain a cold-rolled sheet with a final thickness. The aforementioned cold-rolled sheet is subjected to decarburization annealing, which also serves as primary recrystallization annealing, to obtain a decarburized annealed sheet. A method for manufacturing grain-oriented electrical steel sheets, comprising a series of steps including applying an annealing release agent to the surface of the decarburized annealed sheet and performing finish annealing on the decarburized annealed sheet, (I) When cold rolling is performed once, after hot-rolled sheet annealing and before cold rolling, (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, an 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. A method for manufacturing grain-oriented electrical steel sheets, characterized in that, during the heating process when performing the decarburization annealing on the cold-rolled sheet, a holding treatment is performed such that the heating rate of the cold-rolled sheet is -5°C / s or more and 25°C / s or less, 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), and 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. A method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the heating rate of the cold-rolled sheet during the heating process, excluding the holding treatment, when subjecting the cold-rolled sheet to the decarburization annealing, is 40°C / s or more.

3. The aforementioned component composition is Group A: One or more elements selected by mass%, from Al: 0.005-0.050%, N: 0.003-0.020%, S: 0.002-0.030%, Se: 0.003-0.030%, Sn: 0.005-0.500%, Cr: 0.005-0.500%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Bi: 0.005-0.500%, P: 0.005-0.500%, Sb: 0.005-0.500%, and Mo: 0.005-0.500%. Group B: One or more elements selected from the following in mass percent 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 selected by mass%, from 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%, and Group D: One or more materials selected by mass%, from Ag: 0.001-0.050%, Au: 0.001-0.050%, Ca: 0.001-0.020%, Ga: 0.0001-0.0050%, Ge: 0.0001-0.0050%, Nd: 0.001-0.020%, and La: 0.001-0.020%. A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2, further comprising at least one group of the following.

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