NON-ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Abstract

In the production of a non-oriented electrical steel sheet comprising subjecting a plate containing, as % by mass, C: not more than 0.0050%, Si: from 3.2 to 5.0%, Mn: not more than 2.0%, P: not more than 0.02%, S: not more than 0.0050%, Al: from 0.5 to 2.0% and N: not more than 0.0050%, provided that Si + Al = 4.0%, to hot rolling, hot strip annealing, cold rolling to form a cold-rolled sheet of a final sheet thickness and finish annealing, the reduction by rolling of a final finish rolling pass in hot rolling being not less than 10% and the rolling temperature not greater than 620°C and the soaking temperature in finish annealing being 600 to 800°C to achieve properties of so that a recrystallization ratio is less than 100% as an area ratio, a resistance C is not less than 2.0 and a resistance difference CD is not more than 2.0, where C is a resistance at F= 0° and f1= 0° and D is a resistance at F= 20° and f1= 0° in a section of f2= 45° of the ODF obtained in a central layer of a sheet thickness, thus obtaining a non-oriented electrical steel sheet that has high strength and low iron loss.

Description

NON-ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR PRODUCING THE SAME FIELD OF INVENTION This invention relates to a non-oriented electrical steel sheet and a method for producing the same, and more particularly to a non-oriented electrical steel sheet having high strength and low iron loss and a method for producing the same. BACKGROUND OF THE INVENTION In recent years, the development of motor-driven systems has made it possible to control the frequency of the drive power supply. There has been an increase in motors operating at variable speeds or rotating at high speeds above commercial frequencies. A motor core consists of a fixed stator core in a motor housing and a high-speed rotating rotor core. Centrifugal force is generated on the rotating rotor core. The magnitude of this centrifugal force is proportional to the radius of rotation and proportional to the square of the rotational speed. Consequently, high-strength steel sheets are required as the raw material for medium and large-sized, high-speed rotating rotor cores. IPM motors, used in drive motors for electric vehicles, hybrid vehicles, etc., have a slot around the outer circumference of the rotor. A magnet is embedded in this slot, concentrating the voltage on a narrow bridge section that supports the magnet. Therefore, even in a relatively small motor, the steel sheet used as the rotor core material must have high strength. In a high-speed motor, eddy current is generated due to the high-frequency magnetic flux, which reduces motor efficiency and causes heat generation. Since the generation of a large amount of heat causes magnet demagnetization, the rotor core material must have low iron loss, even in the high-frequency range. Therefore, a steel sheet is required that offers an excellent balance between high strength and durability for the rotor core. One method for achieving high strength includes, for example, adding a large amount of reinforcing element in solid solution, such as silicon, aluminum, and iron-like elements, thereby forming a superior steel alloy. This method can achieve both low iron loss and high strength and is preferred. Another method involves using precipitation strengthening to achieve high strength by forming a superior alloy, but this method has the drawback that the precipitated second-phase grains significantly inhibit grain growth during finishing or stress-generating annealing. Furthermore, as a different method for achieving high strength by forming the superior steel alloy, a widely used method involves increasing strength by making the crystal grains finer.This method involves forming a motor core using a high-strength material and subjecting only the stator core to stress-relief annealing to cause grain growth, resulting in the advantage that the stator core has low iron loss while the rotor core has high strength. Under these circumstances, other methods are proposed to increase the strength of a non-oriented electrical steel sheet. For example, Patent Literatures 1 and 2 propose a znoonn / Lznz / e / YiAi method to achieve high strength using non-recrystallized texture. Also, Patent Literature 3 proposes a method for controlling the configuration of the non-recrystallized texture to suppress variations in tensile strength and fatigue strength. List of Appointments Patent Literature Patent Literature 1: JP-A-2006-1 69611 Patent Literature 2: JP-A-2005-113185 Patent Literature 3: JP-A-2010-090474 BRIEF DESCRIPTION OF THE INVENTION Technical Problem The inventors have produced a non-oriented electrical steel sheet using a superior steel alloy as raw material by applying the method described in Patent Literatures 1 and 2. As a result, they found that these methods suffer from the problem of highly variable fatigue strength. Although the method described in Patent Literature 3 can suppress variations in tensile strength and fatigue strength, the Al and Mn content is limited to a relatively low amount, causing a problem of low specific strength and high iron loss at high frequencies. The invention arose in consideration of the above problems inherent in conventional methods and one objective thereof is to provide a non-oriented electrical steel sheet that has high strength (high tensile strength and high fatigue strength) and low iron loss using a superior steel alloy having high Si and Al contents as raw material and propose an advantageous method for producing it. Solution to the Problem To solve the problems, the inventors conducted several studies, focusing on a combined technique of increasing strength using a superior steel alloy material and increasing strength using a non-recrystallized texture. As a result, they found that, to reliably produce a non-oriented electrical steel sheet with high strength (high tensile strength and high fatigue strength) and low iron loss using the combined technique, it is effective to adjust the texture of a product sheet, thus completing the invention. The invention based on prior knowledge is a non-oriented electrical steel sheet having a chemical composition comprising C: not more than 0.0050% by mass, Si: from 3.2 to 5.0% by mass, Mn: not more than 2.0% by mass, P: not more than 0.02% by mass, S: not more than 0.0050% by mass, Al: from 0.5 to 2.0% by mass, N: not more than 0.0050% by mass, Ti: not more than 0.0030% by mass, Nb: not more than 0.0010% by mass, V: not more than 0.0050% by mass, O: not more than 0.0050% by mass, provided that Si + Al > 4.0% by mass and the remainder being Fe and unavoidable impurities, wherein a recrystallization ratio is less than 100% as an area ratio, a strength C is not less than 2.0 and a resistance difference CD is not less than 2.0, where C is a resistance at Φ = 0° and φι = 0 and D is a resistance at Φ = 20° and φι = 0 in a section of φ2 = 45° of ODF obtained in a central layer of one sheet thickness. The non-oriented electrical steel sheet according to the invention is characterized by znoonn / Lznz / e / YiAi containing one or two selected from Sn and Sb from 0.005 to 0.20% by mass each, in addition to the above chemical composition. The non-oriented electrical steel sheet according to the invention is characterized by containing one or more selected elements of Ca, Mg and REM from 0.0005 to 0.010% by mass each, in addition to the above chemical composition. The non-oriented electrical steel sheet according to the invention is characterized by containing at least one selected from Cr: from 0.01 to 5% by mass, Cu: from 0.01 to 5% by mass, Ni: from 0.01 to 5% by mass, Mo: from 0.0005 to 0.1% by mass, W: from 0.001 to 0.1% by mass, Co: from 0.01 to 5% by mass, As: from 0.001 to 0.05% by mass and B: from 0.0001 to 0.005% by mass, in addition to the above chemical composition. Furthermore, the invention possesses a method for producing a non-oriented electrical steel sheet comprising subjecting a steel plate having a chemical composition described in any of the preceding points to hot rolling, hot strip annealing, a single cold rolling, or two or more cold rollings with an intermediate annealing between each rolling to form a cold-rolled sheet with a final sheet thickness and a finish annealing, characterized in that a roll reduction of a final finish rolling pass in hot rolling is not less than 10%, a coiling temperature after hot rolling is not greater than 620°C, and a soaking temperature in the finish annealing is from 600 to 800°C to achieve those properties such that a recrystallization ratio is less than 100% as an area ratio, the strength C is not less than 2.0, and a strength difference CD is not greater than 2.0, where C is a resistance Φ= 0 and φι = 0° and D is a resistance Φ= 20° and φι= 0° in a section of φ2= 45° of ODF obtained in a central layer of a sheet thickness. The method for producing a non-oriented electrical steel sheet according to the invention is characterized in that a temperature of the steel sheet on one exit side of a first pass in a final cooling of the cold rolling is not less than 80QC. The method for producing a non-oriented electrical steel sheet according to the invention is characterized in that a rolling reduction of the first pass in the final cold rolling of the cold rolling is not less than 30%. Advantageous Effects of the Invention The invention can provide stable, non-oriented electrical steel sheets that have high strength (high tensile strength and high fatigue strength) and low iron loss, which can be favorably used as a raw material for a motor core in a high-speed rotating motor, a drive motor for electric vehicles or hybrid vehicles, etc. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a graph showing the influence of CD on fatigue strength, where C is a strength Φ = 0 and φι = 0° and D is a strength at Φ = 20° and φι = 0° in a φ2 = 45° section of ODF obtained in a central layer of one sheet thickness. znoonn / Lznz / e / YiAi Figure 2 is a graph showing the influence of a rolling reduction of a final finishing rolling pass on hot rolling and a cooling temperature on CD, where C is a resistance Φ= 0° and φι = 0 and D is a resistance Φ = 20° and φι = 0° in a ψ2 = 45° section of ODF obtained in a core layer of one sheet thickness. Figure 3 is a schematic view illustrating a test specimen used in a fatigue test. DETAILED DESCRIPTION OF THE INVENTION An experiment that has been carried out for the development of the invention will be described. A steel sheet having a chemical composition comprising C: 0.0019% by mass, Si: 3.52% by mass, Mn: 0.41% by mass, P: 0.01% by mass, S: 0.0018% by mass, Al: 0.91% by mass, N: 0.0017% by mass, Ti: 0.0008% by mass, Nb: 0.0001% by mass, V: 0.0012% by mass, O: 0.0025% by mass, the remainder being Fe and unavoidable impurities, is melted by a common refining process and formed into a steel raw material (plate) by a continuous casting method. The plate is then heated to a temperature of 1100°C in a gas oven for 30 minutes and is subjected to hot rolling comprising rough rolling and finishing rolling to form a hot-rolled sheet having a sheet thickness of 1.8 mm, which is wound into a coil.In this case, the thickness of the sheet metal bar is 40 mm and the inlet side temperature FET in the finishing roll is 980°C and an outlet side temperature FDT (final rolling temperature) is 820°C and the pass program in the finishing roll and the winding temperature CT are variably modified. A test specimen of the hot-rolled sheet is then taken, subjected to a heat treatment simulating hot strip annealing at 980°C for 30 seconds in a laboratory, pickled, and cold-rolled to form a cold-rolled sheet with a final sheet thickness of 0.30 mm. The cold-rolled sheet is then annealed at 740°C for 20 seconds to form a finished annealed sheet with a recrystallization ratio of 70% as an area ratio. A test specimen is taken from the finished annealed sheet thus obtained and subjected to the following tensile and fatigue tests. In the tensile test, a JIS No. 5 tensile test specimen was used, with the tensile force applied in the rolling direction of the test specimen, and its tensile strength (TS) was measured according to JIS Z2241. In the fatigue test, a fatigue test specimen with the shape and size shown in Figure 3 was used, with the tensile force applied in the rolling direction, and its fatigue strength was measured after 107 repetitions under pulsating tensile loading conditions, a stress ratio of 0.1, and a frequency of 20 Hz. The results showed that the tensile strength (TS) was stable at approximately 620 MPa, while the fatigue strength varied considerably depending on the experimental conditions. Fatigue strength, in this context, refers to the maximum stress under a given applied load of a specific amplitude. Several studies have been conducted to investigate the cause of the variation in fatigue strength, and as a result, it has been clarified that fatigue strength is significantly affected by the texture of the sheet metal. Specifically, the material that has low fatigue strength is high in the vicinity of the {100}<011 > orientation, i.e., (Φ, φι, φ2) - (0°, 0°, 45°) as an Euler angle, while the material that has high fatigue strength is low in the {100} <011> orientation and high resistance in the vicinity of {114} <011> orientation, i.e., (Φ, φι, φζ) - (20°, 0°, 45°) as an Euler angle. Figure 1 shows the relationship between CD and fatigue strength, where C is the strength at Φ = 0° and φι = 0°, and D is the strength at Φ = 20° and φι = 0° in a ψ² - 45° ODF (Crystalline Orientation Distribution Function) section obtained in a core layer of a sheet thickness. As can be seen from Figure 1, fatigue strength increases rapidly when the CD value is no greater than 2.0, while it almost reaches saturation when the CD value is no greater than 1.5. To increase fatigue strength, therefore, CD must be greater than 2.0, and it is preferable that it not be greater than 1.5. The inventors consider the mechanism as follows, although it has not been sufficiently clarified. In the section at ψ2 = 45°, the orientation group in the vicinity of φi = 0° and Φ = from 0 to 50° corresponds to the so-called α fiber, which is known to be difficult to recrystallize. The unrecrystallized texture of the finished annealed sheet is considered to belong primarily to the α fiber. That is, when the recrystallization ratio is the same, the small CD value corresponds to a large orientation dispersion in the unrecrystallized texture of the finished annealed sheet and a small {100}<011 > orientation, which is considered to contribute to the increase in fatigue strength. In other words, the unrecrystallized texture with the large orientation dispersion is considered to hardly cause deformation along a particular crystal face and provides high fatigue strength in a stable manner. Also, [100}<011 >orientation is known as rolling-stable orientation and has the property that the orientation hardly changes during cold rolling. Consequently, to increase the orientation dispersion in fiber a, it is considered effective to decrease the degree of accumulation in {100} <011> orientation as a primary orientation in the hot-rolled sheet stage (before cold rolling) and promote glass rotation in subsequent cold rolling. The inventors have carried out several experiments to study a hot rolling condition to decrease the degree of buildup by {100} <011> orientation in the hot-rolled sheet, i.e., decrease the C — D value. As a result, as shown in Figure 2, it has been found effective to increase the final pass rolling reduction in the hot-rolling finish rolling and also decrease the winding temperature after hot rolling, specifically, to increase the final pass rolling reduction to no less than 10% and decrease the winding temperature to no more than 620°C.Under this condition, the dislocation density and stored energy of the hot-rolled sheet increase and therefore promote recrystallization in the hot-strip annealing in all orientations to thereby randomize the texture, as a result of which the orientation dispersion of the hot-strip annealed sheet is considered to be promoted. As mentioned above, the resistance C at Φ= 0° and φι= 0° in the ψ2 = 45° section of the ODF is related to the non-recrystallized texture and decreases as recrystallization progresses. In the steel sheet according to the invention, where the non-recrystallized texture remains, i.e., where the recrystallization ratio is less than 100% as a ratio of znoonn / Lznz / e / YiAi area, the value of C must not be less than 2.0, preferably not less than 3.0. The invention was developed based on previous new knowledge. The resistance C in the orientation of (Φ, φι, qv) = (0°, 0°, 45°) and the resistance D in the orientation of (Φ, φι, ψ2) = (20°, 0°, 45°) are determined by calculating the ODF using a series expansion method or an ADC method with a plurality of pole figures (preferably no fewer than 3) selected from the pole figures {110}, {200}, {211}, {310}, and so on, obtained by X-ray diffraction of a core layer of polished sheet thickness at 1 / 2 the thickness of the steel sheet. In this respect, the measurements of C and D are not limited to the above methods and can be performed using another well-known method, for example, a texture measurement via EBSP. The reason for limiting the chemical composition of the steel raw material used in the invention will be described below. C: no more than 0.0050% by mass Carbon is an element that forms a carbide and deteriorates the iron content of a product sheet. Contents exceeding 0.0050% by mass have a noticeable adverse effect. Therefore, carbon is limited to no more than 0.0050% by mass to suppress magnetic aging of the product sheet, and preferably no more than 0.0030% by mass. Yes: 3.2 to 5.0% by mass Silicon (Si) has the effect of increasing the specific strength of steel and reducing iron loss. It also has the effect of increasing the strength of steel through solid solution strengthening. The aforementioned effects become noticeable when the Si content is high, and consequently, the Si content is not less than 3.2% by mass according to the invention. However, a Si content exceeding 5.0% by mass makes rolling difficult, so the upper limit for Si is 5.0% by mass. Preferably, the Si content falls within the range of 3.5% to 4.5% by mass. Mn: not more than 2.0% by mass Manganese (Mn) has the effect of increasing the specific strength of steel and reducing iron loss, similar to silicon (Si), and is preferably contained at no less than 0.1% by mass. However, when the Mn content exceeds 2.0% by mass, iron loss is further reduced by carbonitride precipitation, and therefore the Mn content is limited to no more than 2.0% by mass. Thus, Mn is preferably added at 0.1 to 2.0% by mass, and more preferably at 0.3 to 1.0% by mass. P: no more than 0.02% by mass Phosphorus (P) is an element with a high strengthening capacity in solid solution and is used to adjust the hardness of steel. In the superior steel alloy of the invention, a P content exceeding 0.02% by mass makes rolling difficult due to steel embrittlement; therefore, the upper limit for P is 0.02% by mass, preferably no more than 0.01% by mass. S: not greater than 0.0050% by mass Sulfur (S) is a hazardous element that forms fine sulfides and inhibits grain growth, thereby increasing iron loss. In particular, when the S content exceeds 0.0050% by mass, this adverse effect is significant. Consequently, the upper limit is znoonn / Lznz / e / YiAi 0.0050% by mass and, preferably, not greater than 0.0030% by mass. Al: 0.5 to 2.0% by mass Aluminum (Al) has the effect of increasing the specific strength of steel and reducing iron loss, as does silicon (Si). It also increases the strength of steel through solid solution strengthening. However, these effects are small when the Al content is less than 0.5% by mass, while rolling becomes difficult when it exceeds 2.0% by mass. Consequently, the Al content is set within the range of 0.5 to 2.0% by mass, preferably within the range of 0.7 to 1.3% by mass. N: no more than 0.0050% by mass Nitrogen (N) is a hazardous element that increases fine nitride formation, which inhibits grain growth and increases iron loss. In particular, when the N content exceeds 0.0050% by mass, this adverse effect becomes significant. Consequently, the upper limit is 0.0050% by mass, preferably no greater than 0.0030% by mass. Ti: no more than 0.0030% by mass Titanium (Ti) is a hazardous element that precipitates as fine carbonitride, which inhibits grain growth and increases iron loss. In particular, when the Ti content exceeds 0.0030% by mass, this adverse effect becomes significant. Consequently, the upper limit is 0.0030% by mass, preferably no greater than 0.0020% by mass. Note: no more than 0.0010% by mass Nitrogen (Nb) is a hazardous element that precipitates as fine carbonitride, which inhibits grain growth and increases iron loss, similar to titanium (Ti). In particular, when the Nb content exceeds 0.0010% by mass, this adverse effect becomes significant. Therefore, the upper limit is 0.0010% by mass, preferably no greater than 0.0005% by mass. V: no more than 0.0050% by mass Volt (V) is also a hazardous element that precipitates as fine carbonitride, which inhibits grain growth and increases iron loss, similar to titanium (Ti). In particular, when the V content exceeds 0.0050% by mass, the aforementioned adverse effect becomes significant. Consequently, the upper limit is 0.0050% by mass, preferably no greater than 0.0030% by mass. Or: no more than 0.0050% by mass Oxygen (O₂) is a hazardous element that forms oxide-based inclusions which inhibit grain growth and increase iron loss. Specifically, when the O₂ content exceeds 0.0050% by mass, this adverse effect becomes significant. Consequently, the upper limit is 0.0050% by mass, preferably greater than 0.0030% by mass. Si + Al: not less than 4.0% by mass The non-oriented electrical steel sheet according to the invention must contain silicon (Si) and aluminum (Al) such that the total content is not less than 4.0% by mass, in addition to meeting the above chemical composition requirements. When the total Si and Al content is less than 4.0% by mass, it is difficult to consistently ensure high strength and high fatigue resistance. The total Si and Al content is preferably not less than 4.4% by mass. However, when the total Si and Al content exceeds 6.0% by mass, the steel hardens and rolling becomes difficult. znoonn / Lznz / e / YiAi Therefore, the upper limit is preferably approximately 6.0% by mass. The steel raw material used in the invention may contain the following elements, appropriately, in addition to the above ingredients. Sn and Sb: from 0.005 to 0.20% by mass each Tin (Sn) and antimony (Sb) improve the recrystallized texture and reduce iron loss. To achieve this effect, each element must be added at a concentration of at least 0.005% by mass. However, when each element is added at a concentration of no more than 0.20% by mass, the effect becomes saturated. Therefore, tin and antimony should ideally be added within the range of 0.005% to 0.20% by mass, and more preferably within the range of 0.01% to 0.10% by mass. Ca, Mg and REM: from 0.0005 to 0.010% by mass each Calcium, magnesium, and mineral spirits (MS) form stable sulfides, which reduce fine sulfides and improve grain growth and iron loss. To achieve this effect, each element must be added at a concentration of at least 0.0005% by mass. Adding more than 0.010% by mass of each element further impairs iron loss. Therefore, calcium, magnesium, and MS are preferably added within the range of 0.0005% to 0.010% by mass, and ideally within the range of 0.001% to 0.005% by mass. The steel raw material used in the invention may appropriately contain the following elements in addition to those mentioned above. However, these elements contribute significantly to the cost of the raw material, and it is desirable to minimize their addition. Cr: from 0.01 to 5% by mass Cr has the effect of increasing the specific strength of steel and thus reducing iron loss, as does Si. However, Cr has a small strengthening capacity in solid solution compared to Si and Al, and consequently, it is preferable to add it when it is desired to reduce iron loss without impairing rolling properties. When the amount of addition is less than 0.01% by mass, the aforementioned effect cannot be sufficiently achieved, while when it exceeds 5% by mass, the iron loss reduction effect becomes saturated. Therefore, Cr should preferably be added at a concentration of 0.01 to 5% by mass. Cu: from 0.01 to 5% by mass Copper (Cu) has the effect of increasing the specific strength of steel and thus reducing iron loss, as does silicon (Si). However, copper's strengthening capacity in solid solution is small compared to silicon and aluminum (Al), and consequently, it is preferable to add it when it is desired to reduce iron loss without impairing rolling properties. When the amount of addition is less than 0.01% by mass, the aforementioned effect cannot be sufficiently achieved, although when it exceeds 5% by mass, the iron loss reduction effect becomes saturated. Therefore, it is preferable to add copper within the range of 0.01% to 5% by mass. Ni: from 0.01 to 5% by mass Nickel (Ni) is an element that significantly increases the strength of steel through solid solution reinforcement. When the amount added is less than 0.01% by mass, the desired effect is not sufficiently achieved, while when it exceeds 5% by mass, the cost of raw materials increases considerably. Therefore, it is preferable to add Ni using zinc alloys (ZnO2 / LzO2 / E / YiAi). 0.01 to 5% by mass. Mo: from 0.0005 to 0.1% by mass Molybdenum (Mo) has the effect of thickening the carbide and thus reducing iron loss. However, when the amount added is less than 0.0005% by mass, this effect is insufficient, while when it exceeds 0.1% by mass, the iron loss reduction effect becomes saturated. Therefore, it is preferable to add Mo at a concentration of 0.0005 to 0.1% by mass. W: from 0.001 to 0.1% by mass Wood has the effect of thickening the carbide and reducing iron loss, as does molybdenum. However, when the amount added is less than 0.001% by mass, the aforementioned effect is insufficient, while when it exceeds 0.1% by mass, the iron loss reduction effect becomes saturated. Therefore, it is preferable to add wood at a concentration of 0.001 to 0.1% by mass. Co: from 0.01 to 5% by mass Cobalt (Co) is an element that increases the magnetic moment of iron alloys, thereby increasing magnetic flux density and reducing iron loss. However, when the added amount is less than 0.01% by mass, these effects are insufficient, while at amounts exceeding 5% by mass, the raw material cost increases significantly. Therefore, it is preferable to add Co at a concentration of 0.01 to 5% by mass. As: from 0.001 to 0.05% by mass Arsenic (As) is a grain boundary segregation agent and has the effect of improving texture, thereby reducing iron loss. This effect is achieved by adding at least 0.001% by mass. However, As causes grain boundary embrittlement, and this adverse effect becomes particularly noticeable when added at more than 0.05% by mass. Therefore, it is preferable to add As within the range of 0.001 to 0.05% by mass. B: from 0.0001 to 0.005% by mass Boron (B) is a grain boundary segregation agent and has the effect of improving texture, thereby reducing iron loss, similar to arsenic (As). Since B is highly effective at suppressing grain boundary migration, excessive B addition can further inhibit grain growth during stress-relief annealing, leading to increased iron loss. When the addition amount is less than 0.0001% by mass, the aforementioned effect is insufficient, while when it exceeds 0.005% by mass, the adverse effect of suppressing grain boundary mitigation increases. Therefore, it is preferable to add B at a concentration of 0.0001 to 0.005% by mass. In the non-oriented electrical steel sheet according to the invention, the remainder other than the above elements is Fe and unavoidable impurities. A method for producing a non-oriented electrical steel sheet according to the invention will be described. The non-oriented electrical steel sheet according to the invention can be produced by the usual production process of subjecting a steel raw material (plate) having a chemical composition according to the invention to hot rolling, hot strip annealing, single cold rolling, or two or more cold rollings with an intermediate annealing between each cold rolling to form a cold-rolled sheet with a final sheet thickness and a finishing annealing. It is preferable that the raw steel material (plate) be produced by melting a sheet having a chemical composition adapted to the invention by a usual refining process, wherein the molten steel obtained in a converter, electric furnace or the like is subjected to secondary refining in a vacuum degassing device or the like, to adjust to a given chemical composition and then carry out a continuous casting method or ingot making method. The slab is then reheated to a given temperature in a gas furnace or similar and subjected to hot rolling comprising rough rolling and finish rolling to form a hot-rolled sheet. The temperature for reheating the slab is preferably not higher than 1150°C to suppress the solid solution of Al₂, MnS, or similar and prevent subsequent fine precipitation. More preferably, this is within the range of 1000 to 1130°C. When hot rolling is carried out immediately after continuous casting, the slab reheating step may be omitted when the hot rolling temperature can be ensured. A thin-plate foundry, consisting of a continuous casting machine integrated with a rolling mill, may be used, where the hot rolling process can be omitted. Hot rolling after reheating the slab is the most important step of the invention. To randomize the texture after hot strip annealing, which will be described later, by increasing the dislocation density of the hot-rolled sheet and increasing the CD value, the final pass of the finishing roll is performed at a high rolling reduction and a lower coil winding temperature after hot rolling. Specifically, the rolling reduction of the final pass must not be less than 10% and the winding temperature must not be greater than 620°C. When the rolling reduction of the final pass is less than 10% or the winding temperature is greater than 620°C, the dislocation density in the hot-rolled sheet decreases, and the texture after hot strip annealing is not sufficiently randomized.To safely obtain the above effects, it is preferable that the final pass rolling reduction is not less than 14% and the winding temperature is not greater than 590°C. The hot-rolled sheet is then subjected to hot strip annealing, preferably carried out under soaking conditions of 700 to 1100°C for 1 to 300 seconds. As mentioned earlier, increasing the dislocation density of the hot-rolled sheet promotes recrystallization in all orientations during hot strip annealing, thus promoting texture randomization. However, when the soaking temperature in hot strip annealing is below 700°C, the effect of hot strip annealing is insufficient, while when the soaking temperature exceeds 1100°C, the grain size becomes coarse, and cracking frequently occurs on cold rolling. Hot strip annealing is preferably carried out at 800 to 1000°C for 5 to 100 seconds. The hot-rolled sheet after hot strip annealing is subjected to a single cold rolling or two or more cold rollings with an intermediate annealing between each rolling to form a cold-rolled sheet with a final sheet thickness. Cold rolling is also an important step in the invention. To promote the rotation of {100} glass <011> As an initial orientation in other orientations and increases the orientation dispersion of the unrecrystallized structure, it is preferable to carry out the rolling by increasing the temperature of the steel sheet on one side of the exit of the first pass in the final cold rolling to roll to the final sheet thickness and / or carry out the rolling by decreasing the rolling reduction in the first pass, specifically, by increasing the temperature of the steel sheet on the exit side of the first pass to not less than 80°C and / or increasing the rolling reduction in the first pass to not less than 30%, the rotation of the crystal from {100}<011 > to another orientation becomes insufficient.In other words, if the temperature of the steel sheet is higher or the reduction due to rolling is greater, a plurality of sliding systems are more active, thus easily causing the glass to rotate to another orientation and suppressing the accumulation at {100}<011 >. It is preferable that the temperature of the steel sheet on the exit side of the first pass not be less than 100°C and the reduction due to rolling in the first pass not be less than 40%. Furthermore, the temperature of the steel sheet on the exit side of the first pass can be adjusted by raising the temperature of the steel sheet before rolling or by raising the temperature of a coolant or decreasing the amount of coolant. Next, the cold-rolled sheet, which has been cold-rolled to its final thickness, undergoes a finishing annealing process. Finishing annealing is also an important step in the invention. To reduce iron loss by decreasing the dislocation density and to allow the non-recrystallized structure to no longer contribute to strength (i.e., to allow the recrystallization ratio to fall below 100%), it is important to control the soaking temperature during finishing annealing within the range of 600 to 800°C. When the soaking temperature is below 600°C, the dislocation density increases and iron loss becomes too high, while when it exceeds 800°C, recrystallization is promoted, making it difficult to stably retain the non-recrystallized structure and thus hindering the achievement of high strength. The preferred soaking temperature is within the range of 680 to 760°C.The lower limit of the recrystallization ratio of steel sheet after finishing annealing is not particularly well-defined, but an excessively low recrystallization ratio increases iron loss, so it is preferably in the range of 30 to 95%. The recrystallization ratio can be easily measured by polishing a section of the steel sheet face parallel to the rolling direction, etching the face to recover the microstructure, and comparing the ratio of the area occupied by recrystallized grains to the area of ​​the non-recrystallized zone. It is preferable that the steel sheet, after final annealing, be coated with an insulating coating, if necessary. Any known organic, inorganic, or mixed organic / inorganic coating, suitable for the intended purpose, may be used as the insulating coating. The present invention achieves high strength by using a superior steel sheet as raw material and allowing the non-recrystallized structure to remain intact. It is not necessary to perform light rolling to significantly increase iron loss for higher strength. Furthermore, the presence or absence of light rolling can be determined by observing the texture with an optical microscope or EBSP. The non-oriented electrical steel sheet obtained according to the invention has excellent properties such as low iron loss and high strength, and is therefore suitable as a raw material for a rotor core. On the other hand, when used in a stator core, excellent motor efficiency cannot be achieved due to the high iron loss. In this case, it is preferable that the stator core undergo stress-relief annealing after the motor core is manufactured by punching and rolling the steel sheet. In the stator core subjected to stress-relief annealing, recrystallization and grain growth are promoted to reduce iron loss, allowing the motor efficiency to be further increased. This method is suitable for use in a PM motor, which has a particularly low rotor core iron loss ratio. Examples Sheet metal with the chemical composition shown in Table 1, the remainder being Fe and unavoidable impurities, is melted by a standard refining process and formed into a plate by continuous casting. The plate is heated in a gas furnace to 1100°C for 30 minutes and then hot-rolled, consisting of rough rolling and finish rolling, to form a hot-rolled sheet with a thickness of 1.8 mm. The hot-rolled sheet is then hot-strip annealed at 930°C for 30 seconds, pickled, and cold-rolled to form a cold-rolled sheet with a final thickness of 0.3 mm. The cold-rolled sheet is then finished annealed to form a finished product sheet. The conditions for hot rolling, cold rolling, and finish annealing are summarized and shown in Table 2. Samples of the resulting product sheet are then subjected to a tensile test and a fatigue test. In the tensile test, a JIS No. 5 tensile test specimen is taken from the sample, with the tension direction in the rolling direction, to measure the tensile strength (TS) according to JIS Z2241. In the fatigue test, a fatigue test specimen of the shape and size shown in Figure 3 is taken from the sample, with the tension direction in the rolling direction, to measure the fatigue strength after 107 repetitions under a pulsating tensile loading condition, a stress ratio of 0.1, and a frequency of 20 Hz. Test pieces with a width of 30 mm and a length of 280 mm are taken from the L direction (rolling direction) and the C direction (direction perpendicular to the rolling direction) of the previous sample to measure a W10 / 400 iron loss according to JIS C2550-1. In addition, the Wio / 400 iron loss is measured after stress-relieving annealing (SRA) in a N2 atmosphere at 800°C for 2 hours. znoonn / Lznz / e / YiAi Table 1-1 znoonn / Lznz / e / γΐΛΐ znoonn / Lznz / e / γΐΛΐ Table 1-2 znoonn / Lznz / e / γΐΛΐ a 2-1 a 2-2 znoonn / Lznz / e / γΐΛΐ znoonn / Lznz / e / γΐΛΐ Table 2-3 The measurement results are also shown in Table 2. As can be seen from these results, all steel plates produced using the steel raw materials having the chemical composition adapted to the invention under the conditions adapted to the invention have high strength (tensile strength and fatigue strength) and low iron loss.

Claims

1. A non-oriented electrical steel sheet having a chemical composition comprising C: not more than 0.0050% by mass, Si: from 3.2 to 5.0% by mass, Mn: not more than 2.0% by mass, P: not more than 0.02% by mass, S: not more than 0.0050% by mass, Al: from 0.5 to 2.0% by mass, N: not more than 0.0050% by mass, Ti: not more than 0.0030% by mass, Nb: not more than 0.0010% by mass, V: not more than 0.0050% by mass, O: not more than 0.0050% by mass, provided that Si + Al > 4.0% by mass and the remainder being Fe and unavoidable impurities, characterized in that a recrystallization ratio is less than 100% as an area ratio, and a tensile strength C is not less than 2.0 and, a resistance difference CD is not greater than 2.0, where C is a resistance at Φ- 0o and φι = 0o and D is a resistance at Φ- 20° and φι = 0o in a section of φζ= 45° of ODF obtained in a central layer of u sheet thickness.

2. The non-oriented electrical steel sheet according to claim 1, further characterized in that the non-oriented electrical steel sheet contains one or two selected from Sn and Sb from 0.005 to 0.20% by mass each, in addition to the above chemical composition.

3. The non-oriented electrical steel sheet according to claim 1 or 2, further characterized in that the non-oriented electrical steel sheet contains one or more selected from Ca, Mg and REM from 0.0005 to 0.010% by mass each, in addition to the above chemical composition.

4. The non-oriented electrical steel sheet according to any of claims 1 to 3, further characterized in that the non-oriented electrical steel sheet contains at least one selected from Cr: 0.01 to 5% by mass, Cu: 0.01 to 5% by mass, Ni: 0.01 to 5% by mass, Mo: 0.0005 to 0.1% by mass, W: 0.001 to 0.1% by mass, Co: 0.01 to 5% by mass, As: 0.001 to 0.05% by mass and B: 0.0001 to 0.005% by mass, in addition to the above chemical composition.

5. A method for producing a non-oriented electrical steel sheet comprising subjecting a steel plate having a chemical composition described according to any one of claims 1 to 4 to hot rolling, hot strip annealing, a single cold rolling, or two or more cold rollings with an intermediate annealing between each rolling to form a cold-rolled sheet having a final sheet thickness and a finish annealing, characterized in that a roll reduction of a final finish rolling pass in the hot rolling is not less than 10%, a coiling temperature after hot rolling is not greater than 620°C, and a soaking temperature in the finish annealing is from 600 to 800°C to achieve those properties such that a recrystallization ratio is less than 100% as an area ratio, the strength C is not less than 2.0, and a strength difference CD is not greater than 2.0, where C is a resistance Φ= 0° and φι= 0° and D is a resistance Φ= 20° and φι= 0° in a section of φζ= 45° of ODF obtained in a central layer of a sheet thickness. znoonn / Lznz / e / YiAi.

6. The method for producing a non-oriented electrical steel sheet according to claim 5, further characterized in that the temperature of the steel sheet on the exit side of a first pass in a final cold rolling stage is not less than 80°C.

7. The method for producing a non-oriented electrical steel sheet according to claim 5 or 6, further characterized in that the rolling reduction of the first pass in the final cold rolling stage is not less than 30%.