Finish annealing facility for non-oriented electrical steel sheet, finish annealing method and production method of non-oriented electrical steel sheet, and non-oriented electrical steel sheet

Abstract

An annealing facility for continuously performing finish annealing by sequentially passing a steel sheet after cold rolling through an annealing furnace having at least a first zone, a second zone, and a third zone, wherein each zone of the annealing furnace can independently control the gas composition and dew point of the atmosphere, and the second zone is composed of a heating zone, a cracking zone, and a cooling zone in which the atmosphere temperature is 900°C or higher, and the nitrogen content of the atmosphere gas is controlled to be 30 vol% or less and the dew point to be -40°C or lower. In addition, by performing finish annealing using the annealing facility, an electrical steel sheet, preferably a non-oriented electrical steel sheet with low iron loss, is manufactured.

Description

Technology Field

[0001] The present invention relates to a finishing annealing facility suitable for use in the manufacture of electrical steel sheets, a finishing annealing method using said facility, a method for manufacturing electrical steel sheets, and a non-oriented electrical steel sheet having excellent magnetic properties. Background Technology

[0002] Electrical steel sheets, such as non-oriented electrical steel sheets, are generally produced by melting steel adjusted to a predetermined compositional composition, forming a steel material (slab) using a method such as continuous casting, and then hot-rolling the slab to form a hot-rolled sheet. Subsequently, if necessary, hot-rolled sheet annealing is performed on the hot-rolled sheet, followed by one cold rolling or two or more cold rollings with intermediate annealing in between to form a cold-rolled sheet of the final sheet thickness (product sheet thickness). Subsequently, a finishing annealing is performed on the cold-rolled sheet to impart desired magnetic properties, and if necessary, an insulating film is applied to manufacture the sheet.

[0003] As a technique for performing annealing on a steel plate, for example, Patent Document 1 discloses a technique in which, when annealing a steel plate in a continuous annealing furnace having a heating zone, a heat retention zone (cracking zone), and a cooling zone, the atmosphere of each zone is composed of a gas composition in which hydrogen is 1 to 10 vol% and the remainder is nitrogen 90 to 99 vol% and unavoidable impurities. Prior art literature

[0004] International Publication No. 2007 / 043273 The problem to be solved

[0005] However, the inventors recognized that when finishing annealing is performed on a non-oriented electrical steel sheet in the atmosphere disclosed in the above patent document 1, there are many cases where the desired iron loss characteristics are not stably obtained.

[0006] Therefore, the present invention aims to solve the aforementioned problems of the prior art to provide a finishing annealing facility that obtains excellent iron loss characteristics, propose a finishing annealing method for electrical steel sheets using said finishing annealing facility and a method for manufacturing said steel sheets, and to stably provide a non-oriented electrical steel sheet having excellent iron loss characteristics. means of solving the problem

[0007] To solve the above problem, the inventors repeatedly examined the influence of the atmosphere inside the annealing furnace of the finishing annealing facility on the magnetic properties of electrical steel sheets. As a result, they recognized that the reason magnetic properties were unstable in the prior art was due to the nitriding of the steel sheets occurring during finishing annealing. Furthermore, they discovered that the nitriding of the steel sheets can be suppressed by performing finishing annealing under specific atmospheric conditions, and furthermore, that low iron loss of the steel sheets after finishing annealing can be reliably achieved, leading to the development of the present invention.

[0008] The present invention based on the above recognition is,

[0009] [1] An annealing facility for manufacturing electrical steel sheets by sequentially passing a steel sheet after cold rolling through an annealing furnace having at least a first zone, a second zone and a third zone to continuously perform finishing annealing, wherein each zone of the annealing furnace can independently control the gas composition and dew point of the atmosphere, and the second zone is composed of a heating zone, a cracking zone and a cooling zone with an atmosphere temperature of 900°C or higher, and the nitrogen content of the atmosphere gas is controlled to be 30 vol% or less and the dew point to be -40°C or lower.

[0010] [2] In addition, the cooling zone constituting part of the second region in the finishing annealing facility of the present invention is characterized by the cooling speed of the steel plate being controlled to 15℃ / s or less.

[0011] [3] In addition, the first zone in the finishing annealing facility of the present invention is characterized by being a heating zone with an atmosphere temperature of less than 900°C and having a nitrogen content of the atmosphere gas controlled to be 80 vol% or more.

[0012] [4] In addition, the present invention proposes a method for finishing annealing of an electrical steel sheet, characterized by performing finishing annealing on a steel sheet after cold rolling using an annealing facility described in any one of [1] to [3].

[0013] [5] In addition, the present invention proposes a method for manufacturing an electrical steel sheet characterized by performing a finishing annealing on a steel sheet after cold rolling using an annealing facility described in any one of [1] to [3].

[0014] [6] In addition, the present invention relates to a nitrogen content as AlN present in a layer 1 / 20 of the plate thickness from one side surface of a steel plate. s (mass%), the nitrogen content present as AlN in the total plate thickness is N t (mass%), when the plate thickness of the steel plate is t (mm), the above N s , N t and t are in the following (1) expression;

[0015] (t×N t )÷{(t / 10)×N s}≥5 ···(1)

[0016] It is a non-oriented electrical steel sheet characterized by satisfying [specific conditions]. Effects of the invention

[0017] According to the present invention, since the nitriding of the steel sheet occurring during final annealing can be effectively prevented, it becomes possible to stably manufacture a non-oriented electrical steel sheet with low iron loss. Accordingly, according to the present invention, it becomes possible to stably provide a non-oriented electrical steel sheet suitable as a core material for motors such as hybrid automobiles, electric vehicles, vacuum cleaners, high-speed generators, air conditioner compressors, and machine tools. Brief explanation of the drawing

[0018] Figure 1 is a graph showing the effect of the finishing annealing atmosphere on iron loss. Figure 2 is a cross-sectional photograph comparing the difference in the surface layer of the product steel sheet due to the finishing annealing atmosphere. Figure 3 is a graph showing the effect of nitriding of the steel plate surface layer on iron loss. Figure 4 is a graph showing the effect of nitrogen partial pressure in the finishing annealing atmosphere on iron loss. Figure 5 is a graph showing the effect of finishing annealing temperature on the nitriding of the steel plate surface layer. Figure 6 is a graph showing the effect of the dew point of the finishing annealing atmosphere on iron loss. Figure 7 is a graph showing the effect of the cooling rate of the second region of the finishing annealing on iron loss. FIG. 8 is a drawing illustrating one example of an embodiment of the present invention, wherein (a) is an example of the configuration of a finishing annealing facility, (b) is an example of a heat pattern, and (c) is an example of an atmosphere control in the facility of (a) is shown by comparing the present invention with the prior art. FIG. 9 is a drawing illustrating another example of an embodiment of the present invention, wherein (a) is an example of the configuration of a finishing annealing facility, (b) is an example of a heat pattern, and (c) is an example of an atmosphere control in the facility of (a) is shown by comparing the present invention with the prior art. Specific details for implementing the invention

[0019] (Form for carrying out the invention)

[0020] First, I will explain the experiment that served as the impetus for developing the present invention.

[0021] Experiment 1

[0022] A steel material (slab) having a composition of C: 0.0021 mass%, Si: 3.7 mass%, Mn: 0.4 mass%, P: 0.01 mass%, S: 0.0016 mass%, Al: 0.6 mass%, N: 0.0022 mass%, Ti: 0.0014 mass%, Nb: 0.0010 mass%, and O: 0.0025 mass%, with the remainder being Fe and unavoidable impurities, was hot-rolled to produce a hot-rolled plate with a thickness of 1.8 mm. Subsequently, the hot-rolled plate was subjected to hot-rolled plate annealing at 900°C for 30 seconds, followed by acid cleaning, and then cold-rolled to produce a cold-rolled plate with a final thickness of 0.25 mm. Next, the above cold-rolled plate was subjected to finishing annealing at 1000°C × 10 s using a horizontal continuous annealing furnace equipped with a heating zone, a cracking zone, and a cooling zone. At this time, the atmosphere inside the furnace of the cracking zone was set to a mixed gas atmosphere of H2:N2 = 20:80 in vol% ratio (dew point: -55°C) or a 100 vol% Ar gas atmosphere (dew point: -55°C).

[0023] Next, from the steel plate after the above-mentioned finishing annealing, a test specimen measuring 180 mm in length × 30 mm in width was cut with the longitudinal direction oriented in the L direction (rolling direction) and the C direction (direction perpendicular to rolling), and the iron loss W was measured by the Epstein test. 10 / 400 Measured.

[0024] Although the measurement results of the above iron loss are shown in Fig. 1, the test specimens that underwent final annealing under a (H2+N2) mixed gas atmosphere showed high iron loss values. To investigate the cause of this, the cross-section of the test specimens was observed using SEM, and as shown in Fig. 2, in the test specimens where an increase in iron loss was confirmed, finely precipitated AlN was identified in the steel plate surface layer, specifically in the layer from the steel plate surface to 1 / 20 of the plate thickness (hereinafter also referred to as the "1 / 20 plate thickness layer"). From this result, it was estimated that the cause of the high iron loss values ​​was that nitriding occurred during final annealing, and fine nitrides were precipitated in the steel plate surface layer.

[0025] In addition, with respect to the steel sheet after the above-mentioned finishing annealing, the content of N (N as AlN) existing as AlN within a 1 / 20 layer of the sheet thickness by electrolytic extraction s (mass%) and the content of N (N as AlN) present as AlN in the total plate thickness N t (mass%) was analyzed. This analysis value and iron loss W 10 / 400 The relationship is summarized and shown in Fig. 3. From this figure, that is, N existing as AlN within a 1 / 20 layer of plate thickness s Specifically, the above N content (N) existing as AlN in the steel sheet that suppresses an increase in the amount s and N t ) and plate thickness t(mm) are in the following formula (1);

[0026] (t×N t )÷{(t / 10)×N s}≥5 ···(1)

[0027] It was found that the increase in iron loss could be suppressed by satisfying [condition].

[0028] Therefore, the inventors conducted experiments to investigate a method to prevent nitriding during finishing annealing and to suppress the precipitation of fine AlN nitride.

[0029] In addition, as a method to suppress the above-mentioned nitriding, it is considered that the nitrogen content of the atmosphere inside the annealing furnace should be reduced. However, if the nitrogen content of the atmosphere in the heating zone is reduced, the formation of a stable alumina layer on the surface of the steel sheet is inhibited. As a result, nitriding of the steel sheet surface layer in the cracking zone is further promoted, which may lead to a decrease in magnetic properties, or the pickup of rolls inside the furnace may be promoted, causing appearance defects such as surface scratches. Therefore, it is considered necessary to include a certain amount of nitrogen in the atmosphere of at least the heating zone of the annealing furnace. Thus, in the experiment described below, the nitrogen content in the atmosphere inside the furnace of the heating zone was controlled to 80 vol%.

[0030] Experiment 2

[0031] First, the following experiment was conducted to investigate the effect of the partial pressure of nitrogen in the atmosphere of the furnace during final annealing on magnetic properties.

[0032] A steel material (slab) having a composition of C: 0.0021 mass%, Si: 3.7 mass%, Mn: 0.4 mass%, P: 0.01 mass%, S: 0.0016 mass%, Al: 0.6 mass%, N: 0.0022 mass%, Ti: 0.0014 mass%, Nb: 0.0010 mass%, and O: 0.0025 mass%, with the remainder being Fe and unavoidable impurities, was hot-rolled to produce a hot-rolled plate with a thickness of 1.8 mm. Subsequently, the hot-rolled plate was subjected to hot-rolled plate annealing at 900°C for 30 seconds, followed by acid cleaning and cold-rolling to produce a cold-rolled plate with a final thickness of 0.25 mm. Next, the furnace atmosphere of the cracking zone of the above cold-rolled plate was changed to a mixed gas atmosphere of H2 and N2, and finishing annealing was performed by varying the N2 content in the range of 0 to 100 vol%. In addition, the dew point of the furnace atmosphere was controlled to -55°C (constant).

[0033] Next, from the steel plate after the above-mentioned finishing annealing, a test specimen measuring 180 mm in length × 30 mm in width was cut with the longitudinal direction oriented in the L direction (rolling direction) and the C direction (direction perpendicular to rolling), and the iron loss W was measured by the Epstein test. 10 / 400 The results were measured and are shown in Figure 4. From this figure, it can be seen that by reducing the partial pressure of nitrogen in the atmosphere of the crack zone to 30 vol% or less, the increase in iron loss can be suppressed by preventing nitriding.

[0034] Experiment 3

[0035] Next, the following experiment was conducted to investigate the temperature range in which nitriding occurs during final annealing.

[0036] A steel material (slab) having a composition of C: 0.0021 mass%, Si: 3.7 mass%, Mn: 0.4 mass%, P: 0.01 mass%, S: 0.0016 mass%, Al: 0.6 mass%, N: 0.0022 mass%, Ti: 0.0014 mass%, Nb: 0.0010 mass%, and O: 0.0025 mass%, with the remainder being Fe and unavoidable impurities, was hot-rolled to produce a hot-rolled plate with a thickness of 1.8 mm. Subsequently, the hot-rolled plate was subjected to hot-rolled plate annealing at 900°C for 30 seconds, followed by acid cleaning, and then cold-rolled to produce a cold-rolled plate with a final thickness of 0.25 mm. Next, the atmosphere inside the furnace of the above cold-rolled plate was changed to a mixed gas atmosphere of H2:N2 = 20:80 (dew point: -55°C) in a vol% ratio where nitriding occurs, and the cracking temperature was varied in the range of 800 to 1050°C, and a finishing annealing was performed by maintaining the cracking temperature for 10 seconds.

[0037] Subsequently, a test specimen was taken from the steel plate after the above-mentioned finishing annealing, and the content of N (N as AlN) present as AlN within a 1 / 20 layer of plate thickness was determined by electrolytic extraction. sAnalyze (mass%) and the results, the finishing annealing temperature and N s The relationship is shown in Figure 5. From this figure, it was found that the temperature range at which the partial pressure of nitrogen needs to be reduced to prevent nitriding during final annealing is 900°C or higher.

[0038] Experiment 4

[0039] Next, the following experiment was conducted to investigate the effect of the atmosphere of the furnace on the characteristics of the final annealing.

[0040] A steel material (slab) having a composition of C: 0.0021 mass%, Si: 3.7 mass%, Mn: 0.4 mass%, P: 0.01 mass%, S: 0.0016 mass%, Al: 0.6 mass%, N: 0.0022 mass%, Ti: 0.0014 mass%, Nb: 0.0010 mass%, and O: 0.0025 mass%, with the remainder being Fe and unavoidable impurities, was hot-rolled to produce a hot-rolled plate with a thickness of 1.8 mm. Subsequently, the hot-rolled plate was subjected to hot-rolled plate annealing at 900°C × 30 s, followed by acid cleaning and cold-rolling to produce a cold-rolled plate with a final thickness of 0.25 mm. Subsequently, the cold-rolled plate was subjected to finishing annealing at 1000°C × 10 s. At this time, the atmosphere inside the furnace was set to Ar: 100% in the range where the temperature of the atmosphere inside the furnace was 900℃ or higher, and the dew point was varied in the range of -60 to -10℃.

[0041] Next, from the steel plate after the above-mentioned finishing annealing, a test specimen measuring 180 mm in length × 30 mm in width was cut with the longitudinal direction oriented in the L direction (rolling direction) and the C direction (direction perpendicular to rolling), and the iron loss W was measured by the Epstein test. 10 / 400The results were measured and shown in Figure 6 as the relationship between the dew point and iron loss. From this figure, it can be seen that in order to prevent an increase in iron loss due to nitriding, it is necessary to control the dew point of the furnace atmosphere in the furnace atmosphere temperature range of 900°C or higher to -40°C or lower.

[0042] Experiment 5

[0043] Next, the inventors conducted the following experiment to investigate the effect of cooling deformation introduced during cooling as a factor other than the atmosphere that may adversely affect iron loss characteristics during finishing annealing.

[0044] A steel material (slab) having a composition of C: 0.0021 mass%, Si: 3.7 mass%, Mn: 0.4 mass%, P: 0.01 mass%, S: 0.0016 mass%, Al: 0.6 mass%, Ti: 0.0014 mass%, Nb: 0.0010 mass%, O: 0.0025 mass%, N: 0.0022 mass%, with the remainder being Fe and unavoidable impurities, was hot-rolled to produce a hot-rolled plate with a thickness of 1.8 mm. Subsequently, the hot-rolled plate was subjected to hot-rolled plate annealing at 900°C for 30 seconds, followed by acid cleaning, and then cold-rolled to produce a cold-rolled plate with a final thickness of 0.25 mm. Next, the above cold-rolled plate was subjected to a finishing annealing process in which it was cooled after being treated at 1000°C × 30 s with a furnace atmosphere of H2:N2 = 20:80 (dew point: -55°C) at a temperature range of 900°C or higher, where nitriding occurs. At this time, the average cooling rate from the above-mentioned cracking temperature of 1000°C to 900°C was varied in the range of 10 to 15°C / s. Here, the reason the temperature range for varying the cooling rate was set to 900°C or higher is that the cooling rate in this temperature range is thought to have a significant effect on cooling deformation.

[0045] Next, from the steel plate after the above-mentioned finishing annealing, a test specimen measuring 180 mm in length × 30 mm in width was cut with the longitudinal direction oriented in the L direction (rolling direction) and the C direction (direction perpendicular to rolling), and the iron loss W was measured by the Epstein test. 10 / 400 The results were measured and shown in Figure 7 as a relationship with the cooling rate. From this figure, it can be seen that by controlling the average cooling rate in the temperature range of 900°C or higher to 15°C / s or less, the increase in iron loss due to cooling deformation can be suppressed.

[0046] Next, the composition of the steel material (slab) used in the manufacture of the non-oriented electrical steel sheet of the present invention will be described.

[0047] The slab used for manufacturing the non-oriented electrical steel sheet of the present invention preferably contains Si: 2.8 to 6.5 mass%, Mn: 0.1 to 2.0 mass%, and Al: 0.3 to 2.0 mass% as basic components.

[0048] In addition, the above slab may contain at least one selected from P: 0.10 mass% or less, Sn: 0.005 to 0.20 mass%, Sb: 0.005 to 0.20 mass%, Ca: 0.0005 to 0.020 mass%, Mg: 0.0005 to 0.020 mass%, REM: 0.0005 to 0.020 mass%, Cr: 0.01 to 1.0 mass%, Co: 0.01 to 1.0 mass%, Ni: 0.01 to 1.0 mass%, Cu: 0.01 to 1.0 mass%, Mo: 0.001 to 0.1 mass%, and W: 0.001 to 0.1 mass% for the purpose of improving magnetic properties or mechanical properties.

[0049] In addition, C, S, N, Ti, Nb, and O are harmful elements that adversely affect magnetic properties by forming carbonitrides, precipitating, forming oxides, or forming sulfides. Therefore, it is desirable to limit the above elements to C: 0.0050 mass% or less, S: 0.0050 mass% or less, N: 0.0050 mass% or less, Ti: 0.0030 mass% or less, Nb: 0.0030 mass% or less, and O: 0.0050 mass% or less, respectively.

[0050] Next, a method for manufacturing a non-oriented electrical steel sheet according to the present invention will be described.

[0051] The non-oriented electrical steel sheet of the present invention can be manufactured using a conventionally known manufacturing process, wherein a steel material (slab) having the above-mentioned composition is hot-rolled to form a hot-rolled sheet, and if necessary, hot-rolled sheet annealing is performed on the said hot-rolled sheet, followed by one cold rolling or two or more cold rolling steps with intermediate annealing in between to form a cold-rolled sheet of the final sheet thickness (product sheet thickness). Subsequently, a finishing annealing is performed on the said cold-rolled sheet, and an insulating film is formed if necessary. This will be explained in detail below.

[0052] The slab that serves as the material for the non-oriented electrical steel sheet can be manufactured by melting steel having a composition suitable for the present invention as described above using a conventionally known refining process such as a converter, an electric furnace, or a vacuum degassing device, and then using a conventional continuous casting method or an ingot-blowing rolling method. In addition, it may be manufactured as a thin billet with a thickness of 100 mm or less by a direct casting method.

[0053] Next, the above slab is hot-rolled into a hot-rolled plate using a method and conditions that are generally known. In addition, the above slab is typically heated to a predetermined temperature in a furnace before being provided for hot rolling, but it may also be provided for hot rolling immediately after casting without reheating. In addition, in the case of a thin cast slab, it may be hot-rolled, or the hot rolling may be omitted and the process may proceed as is to the subsequent steps.

[0054] The hot-rolled sheet produced above is subjected to hot-rolled sheet annealing as needed. It is preferable to perform this hot-rolled sheet annealing under cracking conditions of 800 to 1100°C × 180 s or less. When the cracking temperature is below 800°C, the effect of hot-rolled sheet annealing is small, and the effect of improving magnetic properties is not sufficiently obtained. On the other hand, when the cracking temperature exceeds 1100°C and the cracking time exceeds 180 s, the crystal grains become coarsened, which promotes brittle fracture (sheet breakage) during cold rolling or impairs productivity, making it disadvantageous in terms of manufacturing costs. A more preferable condition is 850 to 1000°C × 60 s or less.

[0055] In addition, it is preferable that the dew point of the atmosphere in the heating zone and cracking zone during the annealing of the hot-rolled sheet be 0°C or higher and 70°C or lower. If the dew point is below 0°C, the oxide layer on the surface of the steel sheet formed during annealing is easily removed during acid cleaning, and there is a risk that nitriding will be promoted during the finishing annealing of the subsequent process, thereby degrading magnetic properties. On the other hand, if the dew point exceeds 70°C, conversely, an oxide film that is difficult to remove by acid cleaning during annealing is formed, which impairs acid cleaning performance or increases the load during cold rolling, thereby hindering productivity.

[0056] Next, the steel sheet after the above hot rolling or after the hot-rolled sheet annealing is made into a cold-rolled sheet of final thickness by two or more cold rollings with an intermediate annealing in between. The above final thickness, i.e., the product thickness, is preferably 0.30 mm or less from the perspective of reducing iron loss.

[0057] Next, the above cold-rolled sheet undergoes finishing annealing, which is the most important process in the present invention, and, if necessary, an insulating film is applied to make it a product sheet. In the finishing annealing, the cracking temperature is preferably set in the range of 900 to 1200°C from the perspective of growing the crystal grains significantly to reduce iron loss. If the cracking temperature is below 900°C, the growth of the crystal grains becomes insufficient, while if the cracking temperature exceeds 1200°C, the crystal grains become excessively coarsened or become unfavorable in terms of thermal energy cost. More preferably, it is in the range of 1000 to 1100°C. Furthermore, regarding the influence on grain growth during finishing annealing, the cracking temperature is overwhelmingly dominant, and the influence of the cracking time is small. Therefore, regarding the cracking time, it is advisable to set it appropriately considering the length of the annealing furnace and productivity.

[0058] FIG. 8 is a drawing illustrating the finishing annealing conditions of the present invention, wherein (a) at the top shows an example of a finishing annealing facility having a horizontal continuous annealing furnace composed of a heating zone, a cracking zone and a cooling zone, and (b) shows an example of a heat pattern in the finishing annealing facility.

[0059] In addition, the present invention is characterized by dividing the continuous annealing furnace shown in FIG. 8 into three zones in the order in which steel plates are passed: a “first zone” in which the atmosphere temperature inside the furnace of the heating zone is less than 900°C, a “second zone” in which the atmosphere temperature inside the furnace of the heating zone, cracking zone, and cooling zone is 900°C or higher, and a “third zone” in which the atmosphere temperature inside the furnace of the cooling zone is less than 900°C, and controlling the atmosphere in each zone as described below.

[0060] Here, an important aspect of the present invention is that, from the perspective of preventing nitriding (nitriding) of the steel sheet surface during finishing annealing, it is necessary to appropriately control the atmosphere inside the continuous annealing furnace. Specifically, the atmosphere inside the furnace in the second region, that is, the temperature range where the atmosphere temperature is 900°C or higher, must be a gas atmosphere of one type among N2, H2, and noble gases, or a mixed gas atmosphere of two or more types, and furthermore, it is necessary to keep the nitrogen content in said atmosphere 30 vol% or less. For example, it is preferable to use a mixed gas atmosphere with a vol% ratio of H2:N2 = 80:20. In addition, the dew point of the above atmosphere must be -40°C or lower from the perspective of preventing nitriding or oxidation. Preferably, the nitrogen content is 20 vol% or less, and the dew point is -50°C or lower. Furthermore, since the dew point of the gas that can be supplied as an industrial gas is approximately -60°C, the lower limit of the dew point is inevitably approximately -60°C.

[0061] In addition, Figure 8 (c) is a diagram showing a comparison between the furnace atmosphere in the heat pattern of the finishing annealing facility shown in (a) and (b) above and the present invention and the prior art. This diagram shows that in the prior art, the atmosphere in the heating zone and cracking zone was a mixed gas atmosphere of H2:N2 = 20:80 in vol% ratio and the atmosphere in the cooling zone was an N2 gas atmosphere, but in the present invention, regardless of the distinction between the heating zone, cracking zone, and cooling zone, the furnace atmosphere in the region where the furnace atmosphere temperature is 900°C or higher is an atmosphere that satisfies the conditions of having a nitrogen content of 30 vol% or less and a dew point of -40°C or less.

[0062] In addition, in the present invention, it is preferable that the cooling zone at the rear end, which is included in a part of the second region, controls the cooling rate until it is cooled from the cracking temperature to 900°C to 15°C / s or less. As mentioned above, if the cooling rate exceeds 15°C / s, the iron loss increases due to cooling deformation. More preferably, it is 10°C / s or less.

[0063] In addition, in the present invention, the first region located upstream of the second region, i.e., the region where the atmosphere temperature inside the furnace of the heating zone is less than 900°C, is preferably an atmosphere having a nitrogen content of 80 vol% or more and a dew point of -40°C or less, in order to form an alumina layer on the surface of the steel plate that has the function of preventing nitriding in the subsequent temperature range of 900°C or more (second region) or preventing the pickup of rolls inside the furnace. A more preferable nitrogen content is 90 vol% or more.

[0064] In addition, it is preferable that the heating rate of the steel plate in the first region of the finishing annealing facility of the present invention be 100°C / s or higher. This is because setting the rate to 100°C / s or higher reduces the priority of recrystallization of the {111} orientation, and randomizes the orientation of the recrystallized grains, thereby obtaining the effect of increasing the magnetic flux density. Conversely, if the rate is excessively high, it is impossible to correct the shape of the steel plate, and there is a risk that the magnetism will deteriorate due to deformation, or that heating non-uniformity will occur in the plate width direction, causing a large deviation in the iron loss value in the plate width direction. Therefore, it is preferable that the upper limit of the heating rate be approximately 300°C / s.

[0065] In addition, when the heating rate of the steel plate in the first region is 100°C / s or higher, as shown in FIG. 9 (a), it is preferable to configure the first region of the finishing annealing facility with a rapid heating zone and a heating zone with a slower heating rate than the rapid heating zone. It is preferable to perform rapid heating in the rapid heating zone up to 700°C to 750°C, which is the Curie point of the steel plate, and then heat at a lower speed than the rapid heating zone up to 900°C. Furthermore, since the effect of reducing the recrystallization priority of the {111} orientation due to rapid heating is more pronounced in a cold-rolled plate that has not undergone hot-roll annealing than in a cold-rolled plate that has undergone hot-roll annealing, it is preferable to perform rapid heating on the latter.

[0066] In addition, the third region following the second region, that is, the cooling zone where the atmosphere temperature inside the furnace is less than 900°C, has almost no risk of introducing cooling deformation or nitriding, so there are no special restrictions on the atmosphere or cooling rate, and for example, rapid cooling may be performed under a 100% nitrogen gas atmosphere.

[0067] In addition, as described above, the finishing annealing facility of the present invention divides the annealing furnace into a first zone, a second zone, and a third zone with the temperature of the atmosphere inside the furnace at 900°C as a boundary, and since it is necessary to individually control the gas composition and dew point of the atmosphere in each zone, it is preferable that the annealing furnace be divided into multiple sections in the direction of travel of the steel plate, as shown in FIG. 8(a) and FIG. 9(a), so that the atmosphere can be controlled for each section.

[0068] In addition, since H2 gas has good heat transfer properties, including it in the atmosphere gas of the second region reduces uneven cooling in the plate width direction during cooling, thereby suppressing the deterioration of magnetic properties due to cooling deformation. However, since H2 gas is an explosive gas, it is desirable to prevent leakage into the first region or the third region below 900°C. In this regard, as shown in FIG. 8(c) and FIG. 9(c), it is desirable to form zone seals (slits) between the first region and the second region, and between the second region and the third region.

[0069] Example 1

[0070] An example of applying the finishing annealing facility of the present invention to the manufacture of the non-oriented electrical steel sheet of the present invention is described below.

[0071] A steel sheet (cold-rolled sheet) for non-oriented electrical steel sheets having a compositional composition containing the components shown in Table 1 and the remainder consisting of Fe and unavoidable impurities was passed through an annealing facility having a horizontal continuous annealing furnace composed of a heating zone, a cracking zone, and a cooling zone as shown in FIG. 8(a), and a finishing annealing was performed. At this time, the finishing annealing conditions were varied as shown in Table 1, by changing the gas composition and dew point of the atmosphere in the first zone (heating zone with an atmosphere temperature of less than 900°C), the second zone (heating zone, cracking zone, and cooling zone with an atmosphere temperature of 900°C or higher), and the third zone (cooling zone with an atmosphere temperature of less than 900°C), while also varying the cracking conditions (cracking temperature, cracking time) and the cooling rate from the cracking temperature in the second zone to 900°C.

[0072] A test specimen was taken from the steel sheet after finishing annealing obtained in this way, and the content of N (N as AlN) present as AlN within a 1 / 20 thickness layer was determined by electrolytic extraction. s (mass%) and the content of N (N as AlN) present as AlN in the total plate thickness N t (mass%) was analyzed. In addition, from the steel plate after the above-mentioned finishing annealing, a test specimen measuring 180 mm in length × 30 mm in width was cut with the longitudinal direction oriented in the L direction (rolling direction) and the C direction (direction perpendicular to rolling), and the iron loss W was measured by the Epstein test. 10 / 400 Measured.

[0073] The results of the above measurements are listed in Table 1. Additionally, the reference iron loss values ​​shown in Table 1 represent the upper limit of iron loss required for each steel plate. Since iron loss depends significantly on plate thickness, and the required iron loss values ​​vary depending on the plate thickness, the above reference iron loss values ​​are given by the following formula:

[0074] W 10 / 400 (W / kg) = 8 + 20t, where t: plate thickness (mm)

[0075] It was calculated using [the method]. From this result, it was confirmed that by applying the finishing annealing facility of the present invention to the manufacture of non-oriented electrical steel sheets and performing finishing annealing that satisfies the conditions of the present invention, nitriding during finishing annealing is suppressed, and as a result, it is possible to stably manufacture non-oriented electrical steel sheets with low iron loss.

[0076]

[0077] Example 2

[0078] Eight coils of cold-rolled steel sheets (cold-rolled sheets) with a thickness of 0.25 mm were prepared, having a compositional composition of Si: 3.7 mass%, Mn: 0.4 mass%, and Al: 0.6 mass%, with the remainder consisting of Fe and unavoidable impurities, as prepared in Example 1. Four of these coils were passed through the annealing equipment shown in FIG. 8 and finished annealing was performed under the same conditions as steel sheets No. 1 to 4 in Table 1 of Example 1. Additionally, the remaining four coils were passed through the annealing equipment shown in FIG. 9, which had a rapid heating zone installed in the first region, and finished annealing was performed under the same conditions as the four coils, except for rapid heating. Furthermore, details of the finished annealing conditions are shown in Table 2.

[0079] A test specimen was taken from the steel sheet after finishing annealing obtained in this way, and the content of N (N as AlN) present as AlN within a 1 / 20 thickness layer was determined by electrolytic extraction. s (mass%) and the content of N (N as AlN) present as AlN in the total plate thickness N t (mass%) was analyzed. In addition, from the steel plate after the above-mentioned finishing annealing, a test specimen measuring 180 mm in length × 30 mm in width was cut with the longitudinal direction oriented in the L direction (rolling direction) and the C direction (direction perpendicular to rolling), and the iron loss W was measured by the Epstein test. 10 / 400 and magnetic flux density B 50 Measured.

[0080] The results of the above measurements are listed in Table 2. From these results, it was confirmed that by using an annealing facility that forms a rapid heating zone in the first region and performing rapid heating during the heating process of the final annealing, the magnetic flux density can be increased compared to the case where rapid heating is not performed.

[0081]

[0082] Industrial applicability

[0083] The technology of the present invention can be applied not only to iron core materials for drive motors of HEVs, EVs, and FCEVs, but also to the manufacture of iron core materials for motors of air conditioner compressors, machine tools, high-speed generators, vacuum cleaners, etc.

Claims

Claim 1 An annealing facility for manufacturing non-oriented electrical steel sheets by sequentially passing a steel sheet after cold rolling through an annealing furnace having at least a first zone, a second zone, and a third zone to continuously perform finish annealing, wherein each zone of the annealing furnace can independently control the gas composition of the atmosphere and the dew point, and the second zone is composed of a heating zone, a cracking zone, and a cooling zone with an atmosphere temperature of 900°C or higher, and the nitrogen content of the atmosphere gas is controlled to be 30 vol% or less and the dew point to be -40°C or lower. Claim 2 A finishing annealing facility according to claim 1, characterized in that the cooling zone constituting part of the second region is formed by controlling the cooling rate of the steel plate to 15℃ / s or less. Claim 3 A finishing annealing facility according to claim 1, characterized in that the first zone is a heating zone with an atmosphere temperature of less than 900℃ and the nitrogen content of the atmosphere gas is controlled to be 80 vol% or more. Claim 4 A finishing annealing facility according to claim 2, wherein the first zone is a heating zone with an atmosphere temperature of less than 900℃ and the nitrogen content of the atmosphere gas is controlled to be 80 vol% or more. Claim 5 A method for finishing annealing of a non-oriented electrical steel sheet, characterized by performing finishing annealing on a steel sheet after cold rolling using an annealing facility described in any one of claims 1 to 4. Claim 6 A method for manufacturing a non-oriented electrical steel sheet, characterized by performing a finishing annealing on a steel sheet after cold rolling using an annealing facility described in any one of claims 1 to 4.

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