Non-oriented electromagnetic steel sheet

Abstract

This non-oriented electromagnetic steel sheet satisfies the condition that, in a cross-section parallel to the surface of the steel sheet, the surface area ratio of crystal grains having a crystal-grain diameter of less than 200μｍ is 10% or less, when a boundary at which the crystal orientation difference is at least 2° but less than 15° is deemed to be the crystal grain boundary.

Description

[Technical Field]

[0001] The present invention relates to a non-oriented electrical steel sheet. This application claims priority based on Japanese Patent Application No. 2021-061752, filed in Japan on March 31, 2021, and Japanese Patent Application No. 2021-099597, filed in Japan on June 15, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] BACKGROUND ART In recent years, with the global increase in demand for electric devices such as motors, there has been an increase in demand for non-oriented electrical steel sheets used as materials for motors and the like.

[0003] Patent Document 1 describes a method for producing a motor core by punching a non-oriented electrical steel sheet, in which the circularity of the cutting edge of the punching die is controlled in accordance with the elongation percentage of the non-oriented electrical steel sheet. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 10-24333 Summary of the Invention [Problem to be solved by the invention]

[0005] Non-oriented electrical steel sheets used for motor stator cores are punched out with a die, and then core annealing is performed to reduce iron loss. Specifically, the rotor core is first punched out from the non-oriented electrical steel sheet, then the inner diameter of the stator core is punched out, and then the outer diameter of the stator core is punched out. By applying heat to the stator core through core annealing, residual stress and distortion are released, reducing iron loss. While core annealing is performed on the stator core, it is common for rotor cores not to be annealed in order to increase their strength.

[0006] If the residual stress distribution inside the stator core after it has been punched out with a die is uneven, the shape of the stator core will change unevenly as the residual stress is released during core annealing. This can cause the high roundness of the inner diameter of the stator core, which was achieved before core annealing, to deteriorate as a result of core annealing. One of the factors that causes the residual stress distribution to be uneven is the structural unevenness of the non-oriented electrical steel sheet used for punching.

[0007] The technology described in Patent Document 1 controls the circularity of the cutting edge of a punching die according to the elongation percentage of a non-oriented electrical steel sheet, and controls the dimensional accuracy of the processed non-oriented electrical steel sheet by the dimensions of the punching die. For this reason, the technology described in Patent Document 1 does not anticipate providing a non-oriented electrical steel sheet that can achieve desired dimensional accuracy without depending on the dimensions of the punching die.

[0008] Therefore, an object of the present invention is to provide a non-oriented electrical steel sheet that can suppress a decrease in dimensional accuracy after working and the subsequent core annealing (stress relief annealing). [Means for solving the problem]

[0009] The gist of the present disclosure is as follows. (1) In a non-oriented electrical steel sheet according to one embodiment of the present invention, when a boundary with a crystal orientation difference of 2° or more and less than 15° is considered to be a grain boundary in a cross section parallel to the steel sheet surface, the area ratio of crystal grains with a crystal grain size of less than 200 μm satisfies the requirement of 10% or less. (2) In the non-oriented electrical steel sheet described in (1) above, the maximum grain size is D15 when the boundary with a crystal orientation difference of 15° or more is considered to be the grain boundary in a cross section parallel to the steel sheet surface. MAX The average grain size when the boundary with a crystal orientation difference of 2° or more is considered as the grain boundary is D2 AVE When this is the case, the following formula (1) may be satisfied. D15 MAX / D2 AVE ≦5.0 (1) (3) In the non-oriented electrical steel sheet described in (1) or (2) above, when a boundary with a crystal orientation difference of 15° or more is considered to be a grain boundary in a cross section parallel to the steel sheet surface, the shape obtained by approximating the shape of a crystal grain having a grain size of 200 μm or more by an ellipse may satisfy the following formula (2), where DL is the long axis length and DC is the short axis length: DL / DC≦5.0 (2) (4) The non-oriented electrical steel sheet according to any one of (1) to (3) above comprises, in mass%, C: 0% or more and 0.0050% or less, Si: 2.00% or more and 3.25% or less, sol.Al: 0% or more and 1.10% or less, Mn: 0% or more and 1.10% or less, P: 0% or more and 0.30% or less, S: 0% or more and 0.0100% or less, N: 0% or more and 0.0100% or less, Ti: 0% or more and 0.1000% or less, V: 0% or more and 0.100% or less, Zr: 0% or more and 0.100% or less, Nb: 0% or more and 0.100% or less, B: 0% or more and 0.100% or less, O: 0% or more and 0.100% or less, Mg: 0% or more and 0.100% or less, Ca: 0% or more and 0.010% or less, Cr: 0% or more and 5.000% or less, Ni: 0% or more and 5.000% or less, Cu: 0% or more and 5.000% or less, Sn: 0% or more and 0.100% or less, Sb: 0% or more and 0.100% or less, Ce: 0% or more and 0.100% or less, Nd: 0% or more and 0.100% or less, Bi: 0% or more and 0.100% or less, W: 0% or more and 0.100% or less, Mo: 0% or more and 0.100% or less, and Y: 0% or more and 0.100% or less, and the balance being Fe and impurities, The plate thickness is 0.10 mm or more and 0.35 mm or less, The average crystal grain size may be 10 μm or more and 200 μm or less. [Effects of the Invention]

[0010] According to the above-described embodiment of the present invention, it is possible to provide a non-oriented electrical steel sheet that can suppress a decrease in dimensional accuracy after processing and the subsequent core annealing (stress relief annealing). [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a plan view showing a state in which a rotor core is inserted into a stator core. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, a non-oriented electrical steel sheet and a method for manufacturing the same according to an embodiment of the present invention will be described with reference to the drawings.

[0013] The non-oriented electrical steel sheets used for stator cores are punched into the shape of the stator core using a die, and then heat treated (core annealing) to reduce iron loss. By heat treating the steel sheets after punching, any residual stress and distortion is released, reducing iron loss.

[0014] If the residual stress distribution in the stator core after punching the stator core with a die is uneven during core annealing, the shape of the stator core will change unevenly due to the release of the residual stress, which may result in a decrease in the high roundness of the inner diameter of the stator core that was achieved before core annealing.

[0015] FIG. 1 is a plan view showing a state in which a rotor core is inserted inside a stator core. In FIG. 1, stator core 21 is formed by laminating non-oriented electromagnetic steel sheets in a direction perpendicular to the plane of the page, and is composed of a core back 22 on the outer periphery and a plurality of teeth 23 protruding inward from core back 22. Rotor core 30 is inserted inside the tips of teeth 23. Rotor core 30 has a rotor core 31 and a plurality of magnets 32 provided on the outer periphery of rotor core 31 and facing the tips of teeth 23. The outer periphery of stator core 21 is held by a case 50. A shaft 60 passes through the center of rotor core 31 and is fixed to rotor core 31. Shaft 60 is supported by case 50 (or another fixing member) to be rotatable about center O, with the center O of shaft 60 aligned with the center of the inner diameter of stator core 21.

[0016] In FIG. 1 , the inner diameter of the stator core is specifically the inner diameter D of the tips of the teeth 23. If the circularity of this inner diameter D decreases due to the shape change of the stator core 21 caused by the release of residual stress as described above, the gap between the tips of the teeth 23 and the rotor core 31 becomes uneven. This increases the cogging torque when the rotor core 30 rotates. The increased cogging torque can cause uneven rotation, vibration, noise, and other problems. Furthermore, if the circularity of the inner diameter of the stator core 21 decreases further, problems such as the rotor core 31 of the rotor core 30 hitting the tips of the teeth 23 can occur.

[0017] JIS B0621 (1984) "Definition and Display of Geometric Deviation" defines "Circularity refers to the magnitude of deviation of a circular shape from a geometrically correct circle." It also states that circularity is expressed as "When a circular shape is sandwiched between two concentric geometric circles, circularity is expressed as the difference in radius between the two circles with the smallest distance between them, and is expressed as circularity mm or circularity μm."

[0018] In this embodiment, the evaluation criterion for roundness is the ratio obtained by dividing the difference between the maximum and minimum diameters of the circles by the average diameter. The maximum diameter of the circles is the diameter of the larger of the two concentric circles described in JIS B0621 (1984), and the maximum diameter of the circles is the diameter of the smaller of the two concentric circles. The average diameter is the average of the maximum and minimum diameters of the circles. Therefore, the evaluation criterion for roundness in this embodiment corresponds to the ratio obtained by dividing the roundness, which is the difference in radius between the two concentric circles when the distance between them is smallest, by the average radius of the two circles described in JIS B0621 (1984).

[0019] If the ratio of the difference between the maximum and minimum values ​​of inner diameter D to the average diameter exceeds 0.200%, the gap between the tips of teeth 23 and rotor core 31 will become uneven, causing problems such as uneven rotation, vibration, and noise, and also causing problems such as the rotor core 31 of rotor core 30 hitting the tips of teeth 23. Therefore, the ratio of the difference between the maximum and minimum values ​​of inner diameter D to the average diameter is set to 0.200% or less. This ratio is preferably 0.15% or less, and more preferably 0.100% or less. A smaller ratio is preferable, so no lower limit is set.

[0020] One of the factors that causes uneven residual stress distribution in a stator core is the uneven structure of the non-oriented electrical steel sheet used for punching. In particular, if a non-oriented electrical steel sheet contains a mixture of recrystallized and non-recrystallized structures before core annealing, the residual stresses in each structure differ, resulting in differences in the degree of stress release during core annealing. This results in differences in the shape changes of each structure within the stator core. If the recrystallized and non-recrystallized structures are finely dispersed within the non-oriented electrical steel sheet, the loss of roundness of the inner diameter of the stator core after punching is relatively difficult to detect. However, if the recrystallized and non-recrystallized structures are unevenly distributed within the non-oriented electrical steel sheet, the stress release during core annealing differs between the unevenly distributed structures, resulting in a decrease in the roundness of the inner diameter of the stator core after punching, which results in an increase in errors in motor manufacturing control.

[0021] The uneven distribution of recrystallized and unrecrystallized structures is likely to occur when the average grain size of the steel sheet before final cold rolling is relatively large, or when there are many grains larger than the average grain size in the steel sheet before final cold rolling. Note that "before final cold rolling" refers to the period after the final annealing performed before final cold rolling. For example, in the case of a single-pass cold rolling method in which hot-rolled sheet annealing is not performed, the period after hot-rolled coiling corresponds to "before final cold rolling." In the case of a single-pass cold rolling method in which hot-rolled sheet annealing is performed, the period after hot-rolled sheet annealing corresponds to "before final cold rolling." In the case of a double-pass or multiple-pass cold rolling method, the period after intermediate annealing (after the final annealing) corresponds to "before final cold rolling."

[0022] Furthermore, if a large number of {100}<0vw> oriented grains are present before the final cold rolling, the {100}<0vw> oriented grains become coarsely deformed grains elongated in the rolling direction during cold rolling, and remain as coarse unrecrystallized structures during subsequent annealing.

[0023] In the non-oriented electrical steel sheet of this embodiment, when a boundary where the crystal orientation difference is 2° or more and less than 15° is considered to be a grain boundary in a cross section parallel to the steel sheet surface, the area ratio of crystal grains with a crystal grain size of less than 200 μm satisfies 10% or less.

[0024] Here, the crystal orientation mismatch for determining a crystal boundary is limited to a small misorientation of less than 15°. Regions observed as relatively fine structures with grain sizes of less than 200 μm due to such small misorientation are, in other words, regions where crystals with similar crystal orientations are adjacent. It is believed that such regions were generated by cold rolling and annealing due to the presence of the non-uniform regions where the recrystallized and non-recrystallized structures are unevenly distributed. In other words, the small area ratio of the regions where such crystals with small misorientation are adjacent indicates that the uneven distribution of recrystallized and non-recrystallized structures that causes the uneven distribution of residual stress described above did not exist. In this embodiment, this area ratio is limited to 10% or less, preferably less than 5%. The reason for setting the lower limit of the crystal orientation mismatch for determining a crystal grain boundary to 2° or more is that if the crystal orientation mismatch for determining a crystal grain boundary is too small (for example, about 1°), slight distortion of the crystal or the accuracy of the measuring instrument may lead to the misidentification of a non-crystal boundary as a crystal grain boundary.

[0025] The crystal orientation misorientation and the area ratio of crystal grains are measured as follows. First, a sample (crystalline specimen) is taken from a non-oriented electrical steel sheet, with a cross section parallel to the steel sheet surface (a cross section parallel to the rolling direction and thickness direction of the steel sheet) as the observation surface. The observation surface is then polished to a mirror finish. Next, the crystalline specimen is placed at a large inclination in a scanning electron microscope (SEM) and irradiated with an electron beam to obtain an electron backscatter diffraction (EBSD) pattern. This EBSD pattern is continuously collected using a dedicated EBSD detector, and indexing and crystal orientation calculation are performed on the EBSD pattern. Note that crystal structure analysis using the EBSD method is performed at a magnification of 100x, with five fields of view, and a field size of at least 800 μm x 1,000 μm. The obtained data is analyzed using "OIM Analysis Version 7.3.1" (TSL). In this case, a group of points where the crystal orientation difference between adjacent measurement points is below a certain threshold is considered to be a single crystal grain. After all the crystal grains in the observation field have been identified, the crystal orientation difference between adjacent grains and the area of ​​each crystal grain are determined using "OIM Analysis Version 7.3.1" (TSL).

[0026] In this embodiment, in a cross section parallel to the steel sheet surface, the maximum grain size when a boundary with a crystal orientation difference of 15° or more is considered to be a grain boundary is defined as D15 MAX The average grain size when the boundary with a crystal orientation difference of 2° or more is considered as the grain boundary is D2 AVE When it is set as above, it is preferable to satisfy the following formula (1).

[0027] D15 MAX / D2 AVE ≦5.0...Equation (1)

[0028] This ratio (D15 MAX / D2 AVE ) is an index showing how many adjacent crystals with the same crystal orientation as mentioned above are connected and how wide they are spread. Here, D2 AVE The grain boundary that determines the MAX It includes grain boundaries that determine the D2 AVE The grain structure used to calculateMAX This is a crystalline structure in which some of the crystal grains used to calculate the misorientation ratio are further divided by grain boundaries with small misorientation. The smaller this ratio, the more coarse crystal grains, when boundaries with a misorientation of 15° or more are considered to be crystal grain boundaries, are finely divided by crystal grain boundaries with small misorientation of 2° or more but less than 15°. Limiting this ratio to 5.0 or less is an indicator that there is no structure in which recrystallized and unrecrystallized structures are unevenly distributed, which causes the uneven distribution of residual stress as described above. D15 MAX / D2 AVE is preferably 3.0 or less.

[0029] Furthermore, in this embodiment, when a boundary with a crystal orientation difference of 15° or more in a cross section parallel to the steel sheet surface is considered to be a grain boundary, it is preferable that the shape obtained by approximating the shape of a crystal grain having a grain size of 200 μm or more by an ellipse satisfies the following formula (2), where DL is the long axis length and DC is the short axis length.

[0030] DL / DC≦5.0...Formula (2)

[0031] Here, the approximation to an ellipse can be performed, for example, by following the procedure described in "Study on material control and material maintenance methods by strength characteristic evaluation focusing on crystal grain shape" (Harada et al., Abstracts of the Japan Society of Maintenance Engineering Academic Conference, 2nd, p. 150). For example, a test piece measuring 15 mm wide x 10 mm long, with the rolling direction as the longitudinal direction, is taken from the center of the non-oriented electrical steel sheet in the sheet width direction, and the surface of the test piece is polished to approximately half the sheet thickness to create a mirror finish. The mirror-finished sample is observed at 100x magnification using an SEM equipped with EBSD, and EBSD measurement is performed to analyze the crystal structure. Crystal orientation analysis is performed on the data obtained from the EBSD measurement using "OIM Analysis Version 7.3.1" (TSL). If there are multiple crystal grains with a grain size of 200 μm or more within the observation area, the DL / DC is calculated individually and then averaged.

[0032] As described above, this embodiment defines a situation in which coarse crystal grains are finely divided by crystal grain boundaries with a small misorientation of 2° or more and less than 15°. The term "coarse crystal grains" used here refers to crystal grains when boundaries with a misorientation of 15° or more are considered to be crystal grain boundaries. The coarse crystal grains that are divided are elongated by cold rolling during the manufacturing process, and the fine crystal grains that separate them are likely to occur within such elongated regions. In other words, even if coarse crystal grains exist, if they are not elongated, they should be considered to have occurred independently of the structure in which recrystallized and unrecrystallized structures are unevenly distributed, which causes the uneven distribution of residual stress as described above. In other words, this is an indicator that a structure that causes uneven distribution of residual stress that reduces roundness was not present. Preferably, the DL / DC ratio is 3.0 or less.

[0033] The base sheet for obtaining the non-oriented electrical steel sheet of this embodiment preferably has an average grain size of 200 μm or less and a proportion of grains larger than 200 μm of 10% or less of the total grains before final cold rolling. Also, the base sheet for obtaining the non-oriented electrical steel sheet of this embodiment preferably has an average grain size of 200 μm or less and a proportion of grains larger than 200 μm of 10% or less of the total grains before final cold rolling.

[0034] Such an original sheet is a steel sheet in which the uneven distribution of the recrystallized structure and the non-recrystallized structure described above is suppressed, and it is possible to avoid a decrease in roundness during punching and the subsequent annealing.

[0035] The "base sheet of non-oriented electrical steel sheet" in this embodiment can be used as a motor core in its original state. In other words, although it is called a "base sheet" in the configuration of this embodiment, it is a steel sheet that can be expected to be used as is as a non-oriented electrical steel sheet, which is the material for a motor core.

[0036] The above measurements of grain size and crystal orientation are performed using EBSD. Grain boundaries, grain size, and crystal orientation are determined using "OIM Analysis Version 7.3.1" (TSL), and measurements are obtained. Typical measurement conditions are a beam diameter of 1 μm and a crystal orientation likelihood of 10°. When observing, a sufficiently wide area is observed to avoid bias in the data. For example, an area containing 500 or more crystal grains is observed. In particular, if the crystals being observed are determined to be unevenly distributed, care is taken to observe an area large enough to represent the overall situation without bias.

[0037] The chemical composition of the non-oriented electrical steel sheet according to this embodiment contains Si, optionally containing selected elements, and the balance being Fe and impurities. Each element will be described below.

[0038] C: 0% or more and 0.0050% or less C (carbon) is contained as an impurity and is an element that deteriorates magnetic properties. Therefore, the C content is set to 0.0050% or less, preferably 0.0030% or less. Since a low C content is preferable, there is no need to set a lower limit, and the lower limit may be 0%. However, since it is not easy to industrially achieve a content of 0%, the lower limit may be set to more than 0%, or may be set to 0.0010% or more.

[0039] Si: 2.00% or more and 3.25% or less Silicon (Si) is an element effective in increasing the resistivity of steel sheet and reducing iron loss. Therefore, the Si content is set to 2.00% or more. Si is also an element effective in achieving both magnetic properties and mechanical anisotropy in non-oriented electrical steel sheet. In this case, the Si content is preferably more than 2.50%, more preferably 2.70% or more, even more preferably 2.90% or more, and even more preferably 3.00% or more. On the other hand, excessive Si content significantly reduces magnetic flux density. Therefore, the Si content is set to 3.25% or less. The Si content is preferably 3.20% or less, and even more preferably 3.15% or less.

[0040] sol.Al: 0% or more and 1.10% or less Aluminum (Al) is an effective selective element for increasing the resistivity of steel sheet and reducing iron loss, but excessive Al content significantly reduces magnetic flux density. For this reason, the sol. Al content is set to less than 1.10%. There is no need to set a lower limit for sol. Al, and the lower limit may be 0%. However, to more reliably obtain the effects of the above-mentioned action, it is preferable that the sol. Al content be 0.10% or more. Note that sol. Al refers to acid-soluble aluminum.

[0041] Mn: 0% or more and 1.10% or less Manganese (Mn) is an effective selective element for increasing the resistivity of steel sheet and reducing iron loss. However, since Mn has a higher alloying cost than Si or Al, a high Mn content is economically disadvantageous. Furthermore, excessive Mn content significantly reduces magnetic flux density. For this reason, the Mn content is set to 1.10% or less, preferably 0.90% or less. There is no need to set a lower limit for Mn, and the lower limit may be 0%. However, to more reliably obtain the effects of the above-mentioned actions, the Mn content is preferably 0.0010% or more, and more preferably 0.0100% or more.

[0042] P: 0% or more and 0.30% or less P (phosphorus) is an element that is generally contained as an impurity. However, since it has the effect of improving the texture of non-oriented electrical steel sheets and enhancing their magnetic properties, it may be contained as needed. However, since P is also a solid-solution strengthening element, an excessive P content hardens the steel sheet, making cold rolling difficult. For this reason, the P content is set to 0.30% or less. The P content is preferably 0.20% or less. There is no need to set a lower limit for P, and the lower limit may be 0%. However, to more reliably obtain the effects of the above-mentioned actions, the P content is preferably 0.001% or more, and more preferably 0.015% or more.

[0043] S: 0% or more and 0.0100% or less S (sulfur) is contained as an impurity and combines with Mn in the steel to form fine MnS, which inhibits grain growth during annealing and deteriorates the magnetic properties of the non-oriented electrical steel sheet. For this reason, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less, and more preferably 0.0030% or less. Since a low S content is preferable, there is no need to set a lower limit, and the lower limit may be 0%. However, since it is not easy to industrially achieve a content of 0%, the lower limit may be set to 0.0001%.

[0044] N: 0% or more and 0.0100% or less N (nitrogen) is contained as an impurity and combines with Al to form fine AlN, which inhibits the growth of crystal grains during annealing and deteriorates magnetic properties. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0050% or less, and more preferably 0.0030% or less. Since a low N content is preferable, there is no need to set a lower limit, and the lower limit may be 0%. However, since it is not easy to industrially achieve a content of 0%, the lower limit may be 0.0001% or more, more than 0.0015%, or 0.0020% or more.

[0045] Ti: 0% or more, 0.1000% or less Ti (titanium) is an element that is inevitably mixed into steel and can combine with carbon or nitrogen to form precipitates (carbides and nitrides). When carbides or nitrides are formed, these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Furthermore, the carbides and nitrides inhibit grain growth during finish annealing, degrading the magnetic properties of non-oriented electrical steel sheets. Therefore, the Ti content is set to 0.1000% or less. The Ti content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. The Ti content may be 0%. However, because excessively reducing the Ti content can increase manufacturing costs, the Ti content is preferably 0.0005% or more.

[0046] Ca: 0% or more and 0.010% or less Ca (calcium) is an effective selective element for inclusion control because it suppresses the precipitation of fine sulfides (e.g., MnS, CuS) by generating coarse sulfides. Adding an appropriate amount of Ca improves grain growth and magnetic properties (e.g., iron loss). However, excessive Ca content saturates the above-mentioned effects, resulting in increased costs. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably 0.008% or less, and more preferably 0.005% or less. There is no need to set a lower limit for Ca, and the lower limit may be 0%. However, to more reliably obtain the above-mentioned effects, the Ca content is preferably 0.0003% or more. The Ca content is preferably 0.001% or more, and more preferably 0.003% or more.

[0047] Cr: 0% or more and 5.000% or less Cr (chromium) is an optional element that increases resistivity and improves magnetic properties (e.g., iron loss). However, excessive content can decrease saturation magnetic flux density, and the effects of the above-mentioned action saturate, resulting in increased costs. Therefore, the Cr content is set to 5.000% or less. The Cr content is preferably 0.500% or less, and more preferably 0.100% or less. There is no need to set a lower limit for Cr, and the lower limit may be 0%. However, to more reliably obtain the effects of the above-mentioned action, the Cr content is preferably 0.0010% or more.

[0048] Ni: 0% or more and 5.000% or less Ni (nickel) is an optional element that improves magnetic properties (e.g., saturation magnetic flux density). However, if it is contained in excess, the effects of the above-mentioned action saturate, resulting in increased costs. Therefore, the Ni content is set to 5.000% or less. The Ni content is preferably 0.500% or less, and more preferably 0.100% or less. There is no need to set a lower limit for Ni, and the lower limit may be 0%. However, to more reliably obtain the effects of the above-mentioned action, the Ni content is preferably 0.0010% or more.

[0049] Cu: 0% or more and 5.000% or less, Copper (Cu) is an optional element that improves the strength of steel sheets. However, excessive Cu content can reduce the saturation magnetic flux density, and the effects of the above-mentioned functions saturate, resulting in increased costs. Therefore, the Cu content is set to 5.000% or less. The Cu content is preferably 0.100% or less. There is no need to set a lower limit for Cu, and the lower limit may be 0%. However, to more reliably obtain the effects of the above-mentioned functions, the Cu content is preferably 0.0010% or more.

[0050] Sn: 0% or more and 0.100% or less Sb: 0% or more and 0.100% or less Sn (tin) and Sb (antimony) are optional elements that improve the texture of non-oriented electrical steel sheets and enhance their magnetic properties (e.g., magnetic flux density). Therefore, they may be added as needed. However, excessive addition can embrittle the steel, leading to cold-rolling fracture and degrading magnetic properties. Therefore, the Sn and Sb contents are each set to 0.100% or less. There is no need to set lower limits for Sn and Sb; the lower limit may be 0%. However, to ensure the above-described effects, the Sn content is preferably 0.001% or more, and more preferably 0.010% or more. Furthermore, the Sb content is preferably 0.001% or more, more preferably 0.002% or more, more preferably 0.010% or more, and even more preferably more than 0.025%.

[0051] Ce: 0% or more and 0.100% or less Ce (cerium) is a selective element that suppresses the precipitation of fine sulfides (e.g., MnS, CuS) by forming coarse sulfides and oxysulfides, improving grain growth and reducing iron loss. However, excessive Ce content can produce oxides in addition to sulfides and oxysulfides, which can degrade iron loss. Furthermore, the effects of these actions saturate, resulting in increased costs. Therefore, the Ce content is set to 0.100% or less. The Ce content is preferably 0.010% or less, more preferably 0.009% or less, and even more preferably 0.008% or less. There is no need to set a lower limit for Ce, and the lower limit may be 0%. However, to ensure the effects of these actions, the Ce content is preferably 0.001% or more. The Ce content is more preferably 0.002% or more, even more preferably 0.003% or more, and even more preferably 0.005% or more.

[0052] The chemical composition of the non-oriented electrical steel sheet according to this embodiment may contain, in addition to the above elements, optional elements such as B, O, Mg, Ti, V, Zr, Nd, Bi, W, Mo, Nb, and Y. The contents of these optional elements may be controlled based on known knowledge. For example, the contents of these optional elements may be as follows: V: 0% or more and 0.100% or less, Zr: 0% or more and 0.100% or less, Nb: 0% or more and 0.100% or less, B: 0% or more and 0.100% or less, O: 0% or more and 0.100% or less, Mg: 0% or more and 0.100% or less, Nd: 0% or more and 0.100% or less, Bi: 0% or more and 0.100% or less, W: 0% or more and 0.100% or less, Mo: 0% or more and 0.100% or less, and Y: 0% or more and 0.100% or less.

[0053] Moreover, the non-oriented electrical steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.0010% or more and 0.0050% or less, sol.Al: 0.10% or more and less than 1.10% Mn: 0.0010% or more and 1.10% or less, P: 0.0010% or more and 0.30% or less, S: 0.0001% or more and 0.0100% or less, N: more than 0.0015% and less than 0.0100%, Ti: 0.0001% or more and 0.1000% or less, V: 0.0001% or more and 0.100% or less, Zr: 0.0002% or more and 0.100% or less, Nb: 0.0001% or more and 0.100% or less, B: 0.0001% or more and 0.100% or less, O: 0.0001% or more and 0.100% or less, Mg: 0.0001% or more and 0.100% or less, Ca: 0.0003% or more and 0.010% or less, Cr: 0.0010% or more and 5.000% or less, Ni: 0.0010% or more and 5.000% or less, Cu: 0.0010% or more and 5.000% or less, Sn: 0.0010% or more and 0.100% or less, Sb: 0.0010% or more and 0.100% or less, Ce: 0.001% or more and 0.100% or less, Nd: 0.002% or more and 0.100% or less, Bi: 0.002% or more and 0.100% or less, W: 0.002% or more and 0.100% or less, Mo: 0.002% or more and 0.100% or less, and Y: 0.002% or more and 0.100% or less, It is preferable that the compound contains at least one of the following.

[0054] Furthermore, the B content is preferably 0.01% or less, the O content is preferably 0.01% or less, the Mg content is preferably 0.005% or less, the Ti content is preferably 0.002% or less, the V content is preferably 0.002% or less, the Zr content is preferably 0.002% or less, the Nd content is preferably 0.01% or less, the Bi content is preferably 0.01% or less, the W content is preferably 0.01% or less, the Nb content is preferably 0.002% or less, and the Y content is preferably 0.01% or less. Furthermore, the Ti content is preferably 0.001% or more, the V content is preferably 0.002% or more, and the Nb content is preferably 0.002% or more.

[0055] The above-mentioned chemical compositions can be measured by general analytical methods for steel. For example, the chemical composition can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol-Al can be measured by ICP-AES using the filtrate obtained after thermal decomposition of the sample with acid. Si can be measured using the silicon dioxide gravimetric method, C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0056] The above chemical composition is the composition of a non-oriented electrical steel sheet that does not contain an insulating coating or the like. If the non-oriented electrical steel sheet to be measured has an insulating coating or the like on its surface, the insulating coating or the like is removed before measurement. For example, the insulating coating or the like may be removed by the following method. First, the non-oriented electrical steel sheet having the insulating coating or the like is immersed in an aqueous sodium hydroxide solution, an aqueous sulfuric acid solution, and an aqueous nitric acid solution in that order, and then washed. Finally, it is dried with hot air. This results in a non-oriented electrical steel sheet from which the insulating coating or the like has been removed. Alternatively, the insulating coating or the like may be removed by grinding.

[0057] Furthermore, the non-oriented electrical steel sheet of this embodiment may contain Sr, Ba, La, Pr, Zn, and Cd in addition to the above elements. Sr, Ba, La, Pr, Zn, and Cd coarsen sulfides that inhibit grain growth, facilitating grain growth, and are therefore appropriately contained as needed.

[0058] Next, a preferred method for manufacturing the non-oriented electrical steel sheet of this embodiment will be described. A non-oriented electrical steel sheet can be obtained by subjecting steel containing the above chemical components to, for example, steelmaking, hot rolling, hot-rolled sheet annealing, pickling, cold rolling, and annealing. For example, molten steel can be directly converted into a hot-rolled sheet by rapid solidification, or a hot-rolled sheet can be obtained by thin slab casting and continuous hot rolling.

[0059] Hot-rolled sheet annealing is a process in which a hot-rolled sheet with a residual processed structure is heated to 800 to 1050°C to recrystallize and grow grains. This allows for the creation of a texture favorable for magnetic properties after the subsequent cold rolling and annealing. Furthermore, in a composition in which the α-γ transformation does not occur, hot-rolled sheet annealing is sometimes performed to prevent the occurrence of surface irregularities called ridging during cold rolling, but this embodiment does not require hot-rolled sheet annealing.

[0060] After pickling, the hot-rolled sheet (or hot-rolled annealed sheet) is cold-rolled to a predetermined thickness. Cold rolling may be a single-pass cold-rolling method without intermediate annealing, a double-pass cold-rolling method with intermediate annealing, or a multiple-pass cold-rolling method with multiple intermediate annealings. After cold rolling, the sheet is annealed (finish annealing) to produce a non-oriented electrical steel sheet. Note that an insulating coating may be formed by applying an insulating material to the surface of the non-oriented electrical steel sheet and baking it.

[0061] The manufacturing process of the non-oriented electrical steel sheet of this embodiment satisfies the following conditions in particular in order to obtain a metal structure in which the average crystal grain size of the steel sheet before final cold rolling is 200 μm or less and the proportion of crystal grains larger than 200 μm is 10% or less of the total crystal grains.

[0062] To obtain the above-described metal structure before final cold rolling, for example, in the hot-rolled sheet annealing in the case of a single-pass cold rolling method in which hot-rolled sheet annealing is performed, or in the intermediate annealing before final cold rolling in the case of a double-pass or multiple-pass cold rolling method, the average heating rate in the temperature range of 700°C or higher is set to 50°C / sec or higher, the maximum sheet temperature (annealing temperature) is set to 1050°C or lower, and the soaking time is set to 3 minutes or less. By performing annealing that satisfies these conditions, a metal structure can be obtained before final cold rolling in which the average grain size is 200 μm or lower and the proportion of grains exceeding 200 μm is 10% or lower of the total grains. In other words, a steel sheet can be obtained in which the formation of a structure in which recrystallized and unrecrystallized structures, which causes uneven distribution of residual stress, is suppressed before final cold rolling. Preferably, the average heating rate in the temperature range of 700°C or higher is 60°C / sec or higher. Furthermore, the maximum sheet temperature is preferably 1000°C or lower. The soaking time is preferably 2 minutes or less, and more preferably 1 minute or less.

[0063] It is important to avoid the occurrence of non-uniform strain distribution in steel sheets before hot-rolled annealing or before intermediate annealing before final cold rolling. For example, if skin-pass rolling is performed on steel sheets after some recovery and recrystallization, so-called strain-induced grain boundary migration occurs, in which only crystal grains with a specific orientation undergo abnormal grain growth during subsequent annealing, and the proportion of crystal grains larger than 200 μm may account for 10% or more of the total crystal grains. Therefore, in order to avoid the occurrence of non-uniform strain distribution in steel sheets before annealing, it is important to use a reduction of 15% or more in the previous rolling (hot rolling and cold rolling).

[0064] Furthermore, the annealing atmosphere in the hot-rolled sheet annealing or the intermediate annealing before the final cold rolling is a dry nitrogen atmosphere. If the annealing atmosphere is a wet hydrogen atmosphere, the surface layer of the steel sheet is decarburized, resulting in a coarse structure in the surface layer compared to the inner layer. This coarse structure causes non-uniform strain distribution in the steel sheet, so it is preferable to use a dry nitrogen atmosphere for the annealing atmosphere in the hot-rolled sheet annealing or the intermediate annealing before the final cold rolling.

[0065] The average grain size before and after final cold rolling may be measured by the intercept method specified in JIS G0551:2020. For example, the average grain size measured by the intercept method in the thickness direction and rolling direction of a longitudinal cross-sectional structural photograph may be used. An optical microscope photograph may be used as this longitudinal cross-sectional structural photograph, and a photograph taken at a magnification of, for example, 50 times may be used.

[0066] Furthermore, the manufacturing process of the non-oriented electrical steel sheet of this embodiment satisfies the following conditions in particular in order to make the abundance ratio of {100}<0vw> orientation grains in the steel sheet before final cold rolling 10% or less of all crystal grains.

[0067] To obtain a metal structure in which the proportion of {100}<0vw>-oriented grains in steel sheets before final cold rolling is 10% or less of the total grain size, using electromagnetic stirring during continuous casting is effective in preventing the growth of columnar grains and promoting the formation of equiaxed grains, provided the composition does not undergo α-γ transformation. Furthermore, the presence of a large number of specific oxides during solidification of molten steel promotes the formation of equiaxed grains, with the precipitates acting as nuclei. On the other hand, high superheat (molten steel temperature minus solidification temperature) and a high average cooling rate during solidification facilitate the development of columnar grains. Therefore, the formation of equiaxed grains is promoted by reducing the superheat during casting and slowing the average cooling rate. Specifically, the degree of superheat is preferably 15°C or less, more preferably 10°C or less, and even more preferably 5°C or less. Furthermore, the average cooling rate from 900°C or above during the cooling process is set to 20°C / sec (72,000°C / hr) or less. The average cooling rate is preferably 10°C / sec (36,000°C / hr) or less, and even more preferably 5°C / sec (18,000°C / hr) or less. When reheating cast slabs or ingots before hot rolling, the heating temperature is set to 1,100°C or less, and the heating time is set to 2 hours or less, preferably 1 hour or less.

[0068] The abundance ratio of {100}<0vw> orientation grains is measured as follows. Using OIM Analysis Version 7.3.1 (TSL), the area ratio of each grain orientation of interest is extracted (tolerance set to 10°) from the field of view observed using a scanning electron microscope under the following measurement conditions. The extracted area is divided by the area of ​​the observation field to obtain a percentage. This percentage is the area ratio of each grain orientation.

[0069] The detailed conditions for measuring the abundance ratio of {100}<0vw> orientation grains are as follows. Measurement equipment: Scanning electron microscope with electron backscatter diffraction (SEM-EBSD) ·SEM: “JSM-6400” (manufactured by JEOL) EBSD detector: "HIKARI" (TSL) Step spacing: 2μm Measurement target: Center layer (1 / 2 of plate thickness) of Z-surface of steel plate (cut surface of steel plate in the plate thickness direction) Measurement area: 8000μm x 2400μm Grain boundary: The difference in the crystal orientation angle is 15° or more (a continuous region with an angle difference of less than 15° is considered to be one crystal grain)

[0070] The non-oriented electrical steel sheet according to this embodiment has been described above. The thickness of the non-oriented electrical steel sheet according to this embodiment may be 0.35 mm or less, and more preferably 0.30 mm or less. On the other hand, excessive thinning significantly reduces the productivity of the steel sheet and motors, so the thickness is preferably 0.10 mm or more, and more preferably 0.15 mm or more. The non-oriented electrical steel sheet according to this embodiment has an average crystal grain size of 10 μm or more and 200 μm or less.

[0071] The thickness may be measured using a micrometer. If the non-oriented electrical steel sheet to be measured has an insulating coating on its surface, the insulating coating should be removed before the measurement.

[0072] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present disclosure and that provides similar effects is included within the technical scope of the present disclosure. [Example]

[0073] The present disclosure will be specifically described below using examples. Note that the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to the conditions in the examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the disclosure and the purpose is achieved. The underlines in Tables 1, 2 and 3 indicate that the conditions are outside the range of the present invention, that the manufacturing conditions are unfavorable, or that the circularity is unfavorable.

[0074] Example 1 For the steels designated 1F and 1G in Table 1, containing, by mass, 3.00% Si, 0.50% sol. Al, 0.20% Mn, and 0.01% C, along with other unavoidable impurities, were melted and hot-rolled to a thickness of 2 mm. The hot-rolled steel was heated to the annealing temperature (1,000°C to 1,200°C) at the average heating rate shown in Table 1 and annealed for 2 minutes. The annealing atmosphere was a wet hydrogen atmosphere, and the hot-rolled steel was decarburized. A hot-rolled and annealed steel sheet with a thickness of 2 mm and having the metal structure shown in Table 1 was obtained. Furthermore, with regard to symbols 1A to 1E and 1H to 1N in Table 1, steel containing, by mass%, 3.00% Si, 0.50% sol. Al, 0.20% Mn, 0.0020% C, and other unavoidable impurities was melted and finished to a thickness of 2 mm by hot rolling. The hot-rolled sheet was heated to the annealing temperature (1,000°C to 1,200°C) at the average heating rate shown in Table 1 and hot-rolled for 2 minutes. The annealing atmosphere was a dry nitrogen atmosphere. A hot-rolled annealed sheet having a thickness of 2 mm and having the metal structure shown in Table 1 was obtained. The hot-rolled and annealed sheet was then cold-rolled to a thickness of 0.25 mm and annealed at 750°C for 30 seconds (finish annealing) to obtain a non-oriented electrical steel sheet. The cold rolling was a single-pass cold rolling method. The "Base sheet manufacturing conditions" in Table 1 show the annealing temperature, average heating rate, and annealing atmosphere of the hot-rolled and annealed sheet. The average crystal grain size of the base sheet before final cold rolling was also measured and is shown in Table 1. From the obtained non-oriented electrical steel sheet, a disk with a diameter of 60 mm was punched using a die, and stress relief annealing (core annealing) was performed for 2 hours at 750 ° C., and the roundness was measured before and after annealing. In addition, for the disk after core annealing, D15 MAX / D2 AVE , DL / DC, and the area ratio of crystal grains with a grain size of less than 200 μm (indicated as "*1" in Table 1).

[0075] The magnetic properties were evaluated as follows. First, a 16 x 16 mm square sample was cut from the obtained non-oriented electrical steel sheet for magnetic measurement. The cut sample was then subjected to stress relief annealing (core annealing) at 750°C for 2 hours, and the core loss W10 / 400 and magnetic flux density B50 were measured. If the core loss W10 / 400 was 14.0 W / kg or less and the magnetic flux density B50 was 1.65 T or more, the non-oriented electrical steel sheet was judged to have excellent magnetic properties and passed the test. On the other hand, if at least one of the core loss W10 / 400 of 14.0 W / kg or more and the magnetic flux density B50 of 1.65 T or less was satisfied, the magnetic properties were evaluated as poor and the sample was judged to have failed.

[0076] According to the above-mentioned evaluation criteria, discs for which the ratio of the difference between the maximum and minimum diameters to the average diameter was 0.200% or less were deemed to pass (invention examples) and are recorded in Table 1. Discs for which the ratio of the difference between the maximum and minimum diameters to the average diameter was more than 0.200% were deemed to fail (comparison examples) and are recorded in Table 1.

[0077] In Table 1, 1A and 1B were rejected because the average grain size of the steel sheet before final cold rolling exceeded 200 μm, the proportion of grains exceeding 200 μm exceeded 10%, and the circularity after stress relief annealing did not meet the evaluation criteria. 1A and 1B were rejected because the hot-rolled sheet annealing temperatures exceeded 1050°C. This is thought to be why the average grain size of the steel sheet before final cold rolling exceeded 200 μm and the proportion of grains exceeding 200 μm exceeded 10%. Although the proportion of crystal grains over 200 μm in 1C was 10%, the average crystal grain size of the steel sheet before final cold rolling was over 200 μm, and the roundness after stress relief annealing did not meet the evaluation criteria, so it was rejected. It is thought that the average crystal grain size of the steel sheet before final cold rolling exceeded 200 μm because the temperature reached in the hot-rolled sheet annealing of 1C exceeded 1050°C. For 1F and 1G, the average grain size of the steel sheets before final cold rolling was 200 μm or less, but the proportion of grains larger than 200 μm was more than 10%, and the roundness after annealing did not meet the evaluation criteria, so they were rejected. For 1F and 1G, the hot-rolled sheet annealing temperature was 1050°C, but annealing was performed in a wet hydrogen atmosphere, which is thought to be why the proportion of grains larger than 200 μm exceeded 10%. 1I was rejected because the circularity after stress relief annealing was out of standard. It is thought that the average heating rate of the hot-rolled sheet annealing for 1I was 45°C / sec, which resulted in the proportion of crystal grains larger than 200µm exceeding 10%. Steel sheets 1D, 1E, 1H, 1J to 1N were passed because the average grain size of the steel sheets before final cold rolling was 200 μm or less, the proportion of grains larger than 200 μm was 10% or less, and the roundness after annealing met the evaluation criteria. From the above, it can be seen that by setting the hot-rolled sheet annealing temperature to 1050°C or less and using a dry nitrogen atmosphere for the hot-rolled sheet annealing, the average crystal grain size of the steel sheet before final cold rolling is 200 μm or less, and the proportion of crystal grains larger than 200 μm is 10% or less of the total crystal grains, and the desired roundness can be obtained after punching. Furthermore, it can be seen that all of the inventive examples 1D, 1E, 1H, and 1J to 1N achieve low iron loss and sufficient magnetic flux density (good magnetic properties).

[0078] [Table 1]

[0079] Example 2 Steel containing, by mass, 3.00% Si, 0.50% sol. Al, 0.20% Mn, and 0.0020% C, along with other unavoidable impurities, was melted and the deoxidation time was adjusted to vary the oxygen content in the molten steel. The molten steel was poured into a mold, and ingots were produced by varying the superheat and average cooling rate above 900°C as shown in Table 2. The average cooling rate was as shown in Table 2. The ingots were heated at 1,100°C to 1,200°C for 1 to 5 hours and then hot-rolled to a thickness of 1.8 mm. The hot-rolled sheets were then hot-annealed in a dry nitrogen atmosphere at 1,050°C for 2 minutes to obtain hot-rolled and annealed sheets with a thickness of 1.8 mm and the metallographic structure shown in Table 2. Next, the hot-rolled and annealed sheet was cold-rolled to a thickness of 0.20 mm, and then annealed at 750°C for 30 seconds (finish annealing) to obtain a non-oriented electrical steel sheet. Note that the cold rolling was a single-pass cold rolling method. From the obtained non-oriented electrical steel sheets, disks with a diameter of 60 mm were punched out using a die, and stress relief annealing (core annealing) was performed at 750°C for 2 hours, and the roundness before and after annealing was measured. In addition, in the disk after core annealing, as in Example 1, D15 MAX / D2 AVE The DL / DC ratio and the area ratio of crystal grains with a grain size of less than 200 μm (denoted as "*1" in Table 2) were determined. Furthermore, the magnetic properties of the obtained core disks after annealing were determined in the same manner.

[0080] According to the above-mentioned evaluation criteria, those in which the ratio of the difference between the maximum and minimum diameters to the average diameter was 0.200% or less were deemed to pass (invention examples) and recorded in Table 2. Those in which the ratio of the difference between the maximum and minimum diameters to the average diameter was more than 0.200% were deemed to fail (comparison examples) and recorded in Table 2.

[0081] In Table 2, 2A, 2B, 2F, and 2G had a ratio of grains with {100}<0vw> orientation exceeding 10%, and the roundness after stress relief annealing did not meet the evaluation criteria, so they were rejected. For 2A, the heating temperature of the ingot before hot rolling exceeded 1100°C and the heating time exceeded 2 hours, which is thought to be why the proportion of {100}<0vw> orientation grains exceeded 10%. In addition, for 2B and 2F, the ingots were heated for 2 hours, but the heating temperature exceeded 1100°C, which is thought to be why the proportion of {100}<0vw> orientation grains exceeded 10%. In addition, for 2G, the ingot was heated to a temperature of 1100°C for one hour, but the cooling rate during solidification was relatively high (200,000°C / hr.), which is thought to be why the proportion of grains with the {100}<0vw> orientation exceeded 10%. 2C, 2D, 2E, 2H, and 2K-2N were passed because the proportion of {100}<0vw> oriented grains was 10% or less and the roundness after annealing met the evaluation criteria. From the above, it can be seen that by setting the ingot heating temperature to 1100°C or less, the heating time to 1 hour or less, and the cooling rate to 20°C / s (72000°C / hr.) or less, the proportion of {100}<0vw> oriented grains in the steel sheet before final cold rolling will be 10% or less of the total crystal grains, and the desired roundness can be obtained after punching.

[0082] [Table 2]

[0083] Example 3 For the steels designated 1F and 1G in Table 3, containing, by mass, 3.00% Si, 0.50% sol. Al, 0.20% Mn, and 0.01% C, along with other unavoidable impurities, were melted and hot-rolled to a thickness of 2 mm. The hot-rolled steel was heated to the annealing temperature (1,000°C to 1,200°C) at the average heating rate shown in Table 1 and annealed for 2 minutes. The annealing atmosphere was a wet hydrogen atmosphere, and the hot-rolled steel was decarburized. A hot-rolled and annealed steel sheet with a thickness of 2 mm and having the metal structure shown in Table 3 was obtained. Furthermore, with regard to the symbols 1A to 1E and 1H to 1N in Table 3, steel containing, by mass%, 3.00% Si, 0.50% sol. Al, 0.20% Mn, 0.0020% C, and other unavoidable impurities was melted and finished to a thickness of 2 mm by hot rolling. The hot-rolled sheet was heated to the annealing temperature (1,000°C to 1,200°C) at the average heating rate shown in Table 1 and hot-rolled for 2 minutes. The annealing atmosphere was a dry nitrogen atmosphere. A hot-rolled annealed sheet having a thickness of 2 mm and having the metal structure shown in Table 3 was obtained. Next, the hot-rolled and annealed sheet was cold-rolled to a thickness of 0.25 mm and annealed at 750°C for 30 seconds (finish annealing) to obtain a non-oriented electrical steel sheet. The cold rolling was a single-pass cold rolling method. The "Base sheet manufacturing conditions" in Table 3 show the annealing temperature, average heating rate, and annealing atmosphere of the hot-rolled and annealed sheet. The average crystal grain size of the base sheet before final cold rolling was also measured and is shown in Table 3. From the obtained non-oriented electrical steel sheets, disks with a diameter of 60 mm were punched out using a die and subjected to stress relief annealing (core annealing) at 750°C for 2 hours, and the roundness before and after stress relief annealing was measured. In addition, in the disk after core annealing, as in Example 1, D15 MAX / D2 AVE The DL / DC ratio and the area ratio of crystal grains with a grain size of less than 200 μm (denoted by "*1" in Table 3) were determined. Furthermore, the magnetic properties of the obtained core disks after annealing were determined in the same manner.

[0084] According to the above-mentioned evaluation criteria, those in which the ratio of the difference between the maximum and minimum diameters to the average diameter was 0.200% or less were deemed to pass (invention examples) and recorded in Table 3. Those in which the ratio of the difference between the maximum and minimum diameters to the average diameter was more than 0.200% were deemed to fail (comparison examples) and recorded in Table 3.

[0085] In Table 3, 3A to 3H satisfied the evaluation criteria for roundness before stress relief annealing. However, 3J did not satisfy the evaluation criteria for roundness before stress relief annealing, but the steel sheet before final cold rolling had an average crystal grain size of 200 μm or less, and the proportion of crystal grains larger than 200 μm was 10% or less, so the roundness after annealing satisfied the evaluation criteria, and therefore it was passed.

[0086] In 3A and 3B, the average crystal grain size of the steel sheet before final cold rolling was more than 200 μm, and the proportion of crystal grains more than 200 μm was more than 10%, so the roundness after stress relief annealing did not meet the evaluation criteria, and therefore they were rejected. In 3A and 3B, the temperature reached in the hot-rolled sheet annealing was above 1050 °C, so it is thought that the average crystal grain size of the steel sheet before final cold rolling was more than 200 μm, and the proportion of crystal grains more than 200 μm was more than 10%. In 3C, the proportion of crystal grains more than 200 μm was 10%, but the average crystal grain size of the steel sheet before final cold rolling was more than 200 μm, so the roundness after stress relief annealing did not meet the evaluation criteria, and therefore they were rejected. For 3C, the annealing temperature exceeded 1050°C, which is thought to be why the average grain size of the steel sheet before final cold rolling exceeded 200μm. For 3F and 3G, the average grain size of the steel sheet before final cold rolling was 200μm or less, but the proportion of grains exceeding 200μm was more than 10%, and the roundness after annealing did not meet the evaluation criteria, so they were rejected. For 3F and 3G, the temperature reached during hot-rolled sheet annealing was 1050°C, but because annealing was performed in a wet hydrogen atmosphere, it is thought that the proportion of crystal grains over 200 μm in size exceeded 10%. For 3D, 3E, and 3H, the average crystal grain size of the steel sheet before final cold rolling was 200 μm or less, the proportion of crystal grains larger than 200 μm was 10% or less, and the roundness after annealing met the evaluation criteria, so they were accepted. From the above, it can be seen that by setting the temperature reached in the hot-rolled sheet annealing to 1050°C or less and using a dry nitrogen atmosphere for the hot-rolled sheet annealing, the average crystal grain size of the steel sheet before final cold rolling becomes 200 μm or less, and the proportion of crystal grains larger than 200 μm becomes 10% or less of the total crystal grains, and the desired roundness can be obtained after punching. For 3J, the roundness before stress relief annealing did not meet the evaluation criteria, but the average crystal grain size of the steel sheet before final cold rolling was 200 μm or less, the proportion of crystal grains larger than 200 μm was 10% or less, and the roundness after stress relief annealing met the evaluation criteria, so it was passed.

[0087] As described above, the examples of the invention include those in which both the roundness before and after strain relief annealing satisfy the evaluation criteria, and those in which the roundness before strain relief annealing does not satisfy the evaluation criteria but the roundness after strain relief annealing does. Furthermore, the examples of the invention also include those in which the roundness before strain relief annealing satisfies the evaluation criteria, which includes rotor cores that are not subjected to strain relief annealing. Therefore, the examples of the invention are those in which at least one of the roundness before strain relief annealing and the roundness after strain relief annealing satisfies the evaluation criteria.

[0088] [Table 3]

[0089] Example 4 Steel containing the components shown in Tables 4A and 4B in mass percent and other unavoidable impurities was melted and hot-rolled to a thickness of 2 mm. The hot-rolled sheet was annealed at 1,050°C for 2 minutes. The annealing atmosphere was a dry nitrogen atmosphere, and a hot-rolled annealed sheet with a thickness of 2 mm and having the metal structure shown in Tables 4A and 4B was obtained. The hot-rolled and annealed sheets were then cold-rolled to a thickness of 0.25 mm and annealed at 750°C for 30 seconds (finish annealing) to obtain non-oriented electrical steel sheets. The cold rolling was a single-pass cold rolling method. The average grain size and the proportion of grains over 200 μm in size of the original sheets before final cold rolling were measured and are shown in Table 5. From the obtained non-oriented electrical steel sheets, disks with a diameter of 60 mm were punched out using a die and subjected to stress relief annealing (core annealing) at 750°C for 2 hours, and the roundness before and after stress relief annealing was measured. In addition, in the disk after core annealing, D15 MAX / D2 AVE , DL / DC, and the area ratio of crystal grains with a grain size of less than 200 μm (indicated as "*1" in Table 5).

[0090] According to the above-mentioned evaluation criteria, those in which the ratio of the difference between the maximum and minimum diameters to the average diameter was 0.200% or less were deemed to be acceptable (invention examples) and are shown in Table 5.

[0091] From the obtained non-oriented electrical steel sheets, 16 x 16 mm square samples for magnetic measurements were cut out, and stress relief annealing (core annealing) was performed for 2 hours at 750°C, after which the iron loss W10 / 400 and magnetic flux density B50 were measured. If the iron loss W10 / 400 was 14.00 W / kg or less and the magnetic flux density B50 was 1.650 T or more, the non-oriented electrical steel sheet was judged to have excellent magnetic properties and passed the test.

[0092] In Table 5, all of 4A1 to 4S were passed because the average grain size of the steel sheet before final cold rolling was 200 μm or less, the proportion of grains exceeding 200 μm was 10% or less, and the circularity after core annealing met the evaluation criteria. Furthermore, it can be seen that all of 4A1 to 4S, which are examples of the present invention, have low iron loss and sufficient magnetic flux density (good magnetic properties). From the above, it can be seen that according to the present invention, the average crystal grain size of the steel sheet before final cold rolling is 200 μm or less, and the proportion of crystal grains exceeding 200 μm is 10% or less of the total crystal grains, and good circularity and good magnetic properties can be achieved both after punching and core annealing.

[0093] [Table 4A]

[0094] [Table 4B]

[0095] [Table 5] [Explanation of symbols]

[0096] 21 Stator core 22 Coreback 23 Teeth 30 rotor core 31 Rotor core 32 Magnet 50 cases 60 shaft

Claims

1. In mass%, C: 0% or more and 0.0050% or less, Si: 2.00% or more and 3.00% or less, sol. Al: 0% or more and 1.10% or less, Mn: 0.20% or more and 1.10% or less, P: 0% or more and 0.30% or less, S: 0% or more and 0.0100% or less, N: 0% or more and 0.0100% or less, Ti: 0% or more and 0.1000% or less, V: 0% or more and 0.100% or less, Zr: 0% or more and 0.100% or less, Nb: 0% or more and 0.100% or less, B: 0% or more and 0.100% or less, O: 0% or more and 0.100% or less, Mg: 0% or more and 0.100% or less, Ca: 0% or more and 0.010% or less, Cr: 0% or more and 5.000% or less, Ni: 0% or more and 5.000% or less, Cu: 0% or more and 5.000% or less, Sn: 0% or more and 0.100% or less, Sb: 0% or more and 0.100% or less, Ce: 0% or more and 0.100% or less, Nd: 0% or more and 0.100% or less, Bi: 0% or more and 0.100% or less, W: 0% or more and 0.100% or less, Mo: 0% or more and 0.100% or less, and Y: 0% or more and 0.100% or less, and the balance being Fe and impurities, The plate thickness is 0.10 mm or more and 0.35 mm or less, In a longitudinal cross-sectional structural photograph taken at a magnification of 50 times, the average crystal grain size, which is the average value of crystal grain sizes measured in the plate thickness direction and the rolling direction by the intercept method specified in JIS G0551:2020, is 10 μm or more and 200 μm or less, A non-oriented electrical steel sheet, characterized in that, when boundaries with a crystal orientation difference of 2° or more and less than 15° are considered to be grain boundaries in a cross section parallel to the steel sheet surface, the area ratio of crystal grains with a crystal grain size of less than 200 μm satisfies 10% or less.

2. In a cross section parallel to the steel sheet surface, the maximum grain size when the boundary with a crystal orientation difference of 15° or more is considered to be the grain boundary is defined as D15. ＭＡＸ The average grain size when the boundary with a crystal orientation difference of 2° or more is considered to be the grain boundary is D2 ＡＶＥ 2. The non-oriented electrical steel sheet according to claim 1, wherein the following formula (1) is satisfied: D15 ＭＡＸ / D2 ＡＶＥ ≦5.0 ・・・(1)

3. 3. The non-oriented electrical steel sheet according to claim 1, wherein, when a boundary with a crystal orientation difference of 15° or more is considered to be a grain boundary in a cross section parallel to the steel sheet surface, in a shape obtained by approximating a shape of a crystal grain having a grain size of 200 μm or more by an ellipse, where DL is the long axis length and DC is the short axis length, the following formula (2) is satisfied: DL / DC≦5.0...(2)

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