Non-oriented electrical steel sheets and motor cores, and methods for manufacturing them.

Abstract

A non-oriented electromagnetic steel sheet, wherein: the chemical composition of a base material, in mass%, is not more than 0.0050% C, more than 3.70% but not more than 4.60% Si, more than 0.20% but not more than 0.50% Mn, 0.23-0.75% Al, not more than 0.030% P, not more than 0.0018% S, not more than 0.0040% N, 0.005-0.040% Sn, 0-0.040% Sb, the remainder being Fe and impurities, with [4.2≤Si+Al+0.5×Mn≤4.9] being satisfied; the N content [N]s at a position 20 μm from the surface of the base material in the depth direction is not more than 0.0060%; the base material has an average crystal grain size of 10-30 μm; iron loss W10 / 400 is not more than 20.0 W / kg; saturation magnetic flux density is not less than 1.945 T; tensile strength is not less than 680 MPa; and thickness is 0.10-0.30 mm.

Description

[Technical Field]

[0001] This invention relates to non-oriented electrical steel sheets and motor cores, as well as methods for manufacturing them. [Background technology]

[0002] In recent years, global environmental issues have attracted attention, and the demand for energy conservation efforts has increased significantly. In particular, there is a strong demand for increased efficiency in electrical equipment. Therefore, the need for improved magnetic properties in non-oriented electrical steel sheets, which are widely used as core materials for motors and generators, is becoming even stronger. This trend is particularly pronounced in drive motors for electric and hybrid vehicles, as well as in motors for air conditioner compressors.

[0003] The motor cores of the various motors described above consist of a stator, which is the stationary part, and a rotor, which is the rotor. The characteristics required of the stator and rotor that make up the motor core are different from each other. The stator requires excellent magnetic properties (low iron loss and high magnetic flux density), especially low iron loss and high saturation magnetic flux density, while the rotor requires excellent mechanical properties (high strength).

[0004] Because the required characteristics differ between the stator and the rotor, the desired characteristics can be achieved by manufacturing separate non-oriented electrical steel sheets for the stator and the rotor. However, preparing two types of non-oriented electrical steel sheets leads to a decrease in yield. Therefore, in order to achieve the high strength required for the rotor while also achieving low iron loss, non-oriented electrical steel sheets that are superior in both strength and magnetic properties have been considered for a long time.

[0005] For example, Patent Documents 1 to 4 describe attempts to achieve excellent magnetic properties and high strength. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2019 / 017426 [Patent Document 2] International Publication No. 2020 / 091039 [Patent Document 3] International Publication No. 2020 / 091043 [Patent Document 4] Japanese Patent Publication No. 2008-50686 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, achieving non-oriented electrical steel sheets that combine high strength and low iron loss requires the inclusion of large amounts of alloying elements, as disclosed in Patent Documents 1 to 4. This presents the problem of reduced toughness and increased susceptibility to fracture during cold rolling. Furthermore, increasing the alloying level can sometimes lead to a decrease in saturation magnetic flux density.

[0008] This invention was made to solve these problems and aims to provide a stable supply of non-oriented electrical steel sheets with high strength and excellent magnetic properties. [Means for solving the problem]

[0009] The present invention is essentially based on the following non-oriented electrical steel sheets, motor cores, and methods for manufacturing them.

[0010] (1) The chemical composition of the base material is, in mass%, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75% P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050% Nb: Less than 0.0050% Less than 0.0050% Zr, Less than 0.0050% V, Less than 0.200% Cu, Less than 0.500% Ni, 0.005 - 0.040% Sn, 0 - 0.040% Sb, The balance is Fe and impurities, satisfying the following formula (i), The N content [N]s from the surface of the base material to a position 20 μm in the depth direction is 0.0060% or less, The average crystal grain size of the base material is 10 - 30 μm, Iron loss W 10 / 400 is 20.0 W / kg or less, The saturation magnetic flux density is 1.945 T or more, The tensile strength is 680 MPa or more, The plate thickness is 0.10 - 0.30 mm, Non - oriented electrical steel sheet. 4.2 ≦ Si + Al + 0.5×Mn ≦ 4.9 ···(i) However, the element symbols in the above formula are the contents (mass %) of each element.

[0011] (2) The non - oriented electrical steel sheet according to (1) above having an insulating film on the surface of the base material. The non - oriented electrical steel sheet according to (1) above.

[0012] (3) A method for manufacturing the non - oriented electrical steel sheet according to (1) or (2) above, by mass %, C: 0.0050% or less, Si: More than 3.70% and 4.60% or less, Mn: More than 0.20% and 0.50% or less, Al: 0.23 - 0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050%, Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: less than 0.200% Ni: Less than 0.500% Sn: 0.005~0.040%, Sb: 0~0.040%, The remainder consists of Fe and impurities. For a steel ingot having a chemical composition satisfying the following equation (i), The process involves a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process at a soaking temperature of 650-780°C for 8-36 hours, a cold rolling process to reduce the sheet thickness to 0.10-0.30 mm, and a finish annealing process at a soaking temperature of 700-830°C for 1 second to 10 minutes, in that order. A method for manufacturing non-oriented electrical steel sheets. 4.2≦Si+Al+0.5×Mn≦4.9 (i) However, the element symbols in the above formula represent the content (mass %) of each element. (4) A motor core in which multiple non-oriented electrical steel sheets are laminated, The chemical composition of the base material of the aforementioned non-oriented electrical steel sheet is, in mass%, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75% P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050% Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: less than 0.200% Ni: Less than 0.500% Sn: 0.005~0.040%, Sb: 0~0.040%, The remainder consists of Fe and impurities. The following equation (i) is satisfied, The N content [N]s from the surface of the base material to a depth of 20 μm is 0.0070% or less. The average grain size of the base material is 50 to 120 μm. The saturation magnetic flux density is 1.945T or higher. The thickness of the aforementioned non-oriented electrical steel sheet is 0.10 to 0.30 mm. Motor core. 4.2≦Si+Al+0.5×Mn≦4.9 (i) However, the element symbols in the above formula represent the content (mass %) of each element.

[0013] (5) Having an insulating coating on the surface of the base material, The motor core described in (4) above.

[0014] (6) A method for manufacturing the motor core described in (4) or (5) above, In mass%, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75% P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050% Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: less than 0.200% Ni: Less than 0.500% Sn: 0.005~0.040%, Sb: 0~0.040%, The remainder consists of Fe and impurities. For a steel ingot having a chemical composition satisfying the following equation (i), The process involves the following steps in order: hot rolling, pickling, batch-type hot-rolled sheet annealing at a soaking temperature of 650-780°C for 8-36 hours, cold rolling to reduce the sheet thickness to 0.10-0.30 mm, finish annealing at a soaking temperature of 700-830°C for 1 second to 10 minutes, punching, lamination, and stress-relieving annealing at a soaking temperature of 750-900°C for 10-180 minutes. A method for manufacturing a motor core. 4.2≦Si+Al+0.5×Mn≦4.9 (i) However, the element symbols in the above formula represent the content (mass %) of each element. [Effects of the Invention]

[0015] According to the present invention, a non-oriented electrical steel sheet having high strength and excellent magnetic properties can be obtained. [Modes for carrying out the invention]

[0016] As a result of diligent research conducted by the inventors to solve the above problems, we have obtained the following findings.

[0017] To achieve high strength, low iron loss, and high saturation magnetic flux density while ensuring toughness during cold rolling, it is necessary to optimize the content of the main alloying elements, Si, Mn, and Al.

[0018] Specifically, the mixture contains Si in a concentration of over 3.70% to 4.60%, as Si has the highest solid solution strengthening ability and contributes most to the increase in electrical resistance. In addition, to obtain good grain growth properties, it contains 0.23% or more Al. On the other hand, to suppress the deterioration of the saturation magnetic flux density, the Al content is kept to 0.75% or less. Furthermore, although Mn has the lowest solid solution strengthening ability among the three elements, it contributes to the increase in electrical resistance with minimal toughness deterioration, and therefore contains over 0.20% Mn.

[0019] Through repeated investigations, the inventors found that nitriding of the surface layer of the steel sheet degrades iron loss. Although the mechanism is not yet clear, they discovered that Mn influences the nitriding of the surface layer of the steel sheet. To suppress the degradation of iron loss due to nitriding of the surface layer of the steel sheet, the Mn content is set to 0.50% or less. Furthermore, it was found that Sn also has the effect of suppressing nitriding of the surface layer of the steel sheet. Therefore, Sn is included at a concentration of 0.005-0.040%.

[0020] In typical continuous hot-rolled sheet annealing, scale-covered hot-rolled sheets are inserted into the annealing furnace, resulting in the formation of scale that is difficult to remove during pickling after annealing. Therefore, mechanical descaling, such as shot blasting, is necessary before pickling. However, with high-alloy steels as mentioned above, shot blasting causes twinning deformation on the surface of the steel sheet, and this twinning deformation can easily lead to problems such as sheet fracture and edge cracking during cold rolling.

[0021] In this invention, the hot-rolled sheet is pickled before annealing, and then annealed in a batch furnace. Since the scale on the hot-rolled sheet can be easily removed by pickling, shot blasting is unnecessary and twinning deformation does not occur. Therefore, good toughness can be ensured even with high-alloy steel, and the occurrence of problems such as sheet fracture and edge cracking during cold rolling can be suppressed.

[0022] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0023] 1. Overall Structure The non-oriented electrical steel sheet according to one embodiment of the present invention has high strength and excellent magnetic properties, making it suitable for both stators and rotors.

[0024] Furthermore, when using non-oriented electrical steel sheets as the stator core, reducing iron loss is of particular importance. In such cases, it is preferable to manufacture the motor core by punching out and laminating the non-oriented electrical steel sheets, and then to perform stress-relieving annealing only on the stator core. Since the stator core that has undergone stress-relieving annealing has reduced iron loss due to the advancement of grain growth, the motor efficiency can be greatly improved.

[0025] A motor core according to one embodiment of the present invention is obtained by sequentially performing a punching process, a lamination process, and a stress-relieving annealing process on the non-oriented electrical steel sheet. That is, a motor core according to one embodiment of the present invention is made up of multiple non-oriented electrical steel sheets laminated together. In the following description, the base material of the non-oriented electrical steel sheet constituting the motor core is simply referred to as the "base material of the motor core."

[0026] Furthermore, it is preferable that the non-oriented electrical steel sheet and motor core according to this embodiment have an insulating coating on the surface of the base material, as described below.

[0027] 2. Chemical composition of the base material The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0028] C: 0.0050% or less Carbon (C) is an element that causes iron loss degradation in non-oriented electrical steel sheets. If the C content exceeds 0.0050%, the iron loss of the non-oriented electrical steel sheet deteriorates, and good magnetic properties cannot be obtained. Therefore, the C content should be 0.0050% or less. Preferably, the C content is 0.0040% or less, more preferably 0.0035% or less, and even more preferably 0.0030% or less. Furthermore, since C contributes to increasing the strength of non-oriented electrical steel sheets, if this effect is to be obtained, the C content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0029] Si: more than 3.70% and less than 4.60% Silicon (Si) is an element that increases the electrical resistance of steel, reduces eddy current loss, and improves high-frequency iron loss in non-oriented electrical steel sheets. Furthermore, because Si has a large solid solution strengthening capacity, it is also an effective element for increasing the strength of non-oriented electrical steel sheets. To obtain these effects, the Si content should be greater than 3.70%. Preferably, the Si content is 3.80% or more, more preferably 3.90% or more, and even more preferably 4.00% or more. On the other hand, if the Si content is excessive, the workability deteriorates significantly, making it difficult to carry out cold rolling. Therefore, the Si content should be 4.60% or less. Preferably, the Si content is 4.50% or less, and more preferably 4.40% or less.

[0030] Mn: More than 0.20% and less than 0.50% Manganese (Mn) is an effective element for increasing the electrical resistance of steel, reducing eddy current losses, and improving high-frequency iron loss in non-oriented electrical steel sheets. However, if the Mn content is too low, the effect of increasing electrical resistance is small, and iron loss deteriorates due to the precipitation of fine sulfides (MnS) in the steel. Therefore, the Mn content should be greater than 0.20%. Preferably, the Mn content is 0.25% or more, and more preferably 0.30% or more. On the other hand, if the Mn content is excessive, nitriding of the surface layer of the steel sheet becomes excessive, and iron loss deteriorates. Therefore, the Mn content should be 0.50% or less. Preferably, the Mn content is 0.45% or less, and more preferably 0.40% or less.

[0031] Al: 0.23~0.75% Aluminum (Al) is an element that reduces eddy current losses by increasing the electrical resistance of steel, thereby improving high-frequency iron loss in non-oriented electrical steel sheets. Al also improves iron loss by improving the texture. Furthermore, although not to the same extent as Si, Al contributes to increasing the strength of non-oriented electrical steel sheets through solid solution strengthening. Additionally, the addition of an appropriate amount of Al suppresses the refinement of AlN, which is formed by AlN bonding with N in the steel, improving grain growth during finish annealing and stress-relieving annealing, and suppressing iron loss degradation caused by the fine AlN itself.

[0032] To obtain these effects, the Al content should be 0.23% or higher. Preferably, the Al content should be 0.25% or higher, and more preferably 0.27% or higher. Note that the nitriding of the surface layer of the steel sheet, as mentioned above, is more likely to occur as the Al content increases. As a result, iron loss deteriorates. However, in this invention, by controlling the content of Mn and Sn, nitriding of the surface layer of the steel sheet can be suppressed even when the Al content is high. Therefore, the effects of this invention are particularly pronounced when the Al content is high. That is, the effects of this invention are more pronounced when the Al content is, for example, greater than 0.45% or 0.47% or higher. On the other hand, if the Al content is excessive, the toughness deteriorates and the risk of fracture during cold rolling increases. Therefore, the Al content should be 0.75% or less. Preferably, the Al content should be 0.70% or less, and more preferably 0.65% or less.

[0033] In this embodiment, the electrical resistance of the steel is ensured by appropriately controlling the content of Si, Al, and Mn. Furthermore, from the viewpoint of ensuring strength, it is also necessary to appropriately control the content of Si, Al, and Mn. On the other hand, an upper limit is also necessary from the viewpoint of ensuring saturation magnetic flux density and toughness. Therefore, in addition to the content of Si, Al, and Mn being within the above ranges, it is necessary to satisfy the following equation (i). The value of the middle side of the following equation (i) is preferably 4.3 or more, more preferably 4.4 or more, preferably 4.8 or less, and more preferably 4.7 or less.

[0034] 4.2≦Si+Al+0.5×Mn≦4.9 (i) However, the element symbols in the above formula represent the content (mass %) of each element.

[0035] P:0.030% or less Phosphorus (P) is present in steel as an impurity, and if its content is excessive, the toughness of non-oriented electrical steel sheets will be significantly reduced. Therefore, the P content should be 0.030% or less. Preferably, the P content is 0.025% or less, and more preferably 0.020% or less. However, since extreme reduction of the P content may lead to an increase in manufacturing costs, preferably, the P content is 0.003% or more, and more preferably 0.005% or more.

[0036] S: 0.0018% or less S (sulfur) is an element that increases iron loss by forming fine precipitates of MnS, thereby degrading the magnetic properties of non-oriented electrical steel sheets. Therefore, the sulfur content should be 0.0018% or less. Preferably, the sulfur content should be 0.0016% or less, and more preferably 0.0014% or less. However, since extreme reduction of the sulfur content may lead to increased manufacturing costs, preferably, the sulfur content should be 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0037] N: 0.0040% or less Nitrogen (N) is an element that is inevitably mixed into steel, and it forms nitrides, increasing iron loss and degrading the magnetic properties of non-oriented electrical steel sheets. Therefore, the N content should be 0.0040% or less. Preferably, the N content is 0.0030% or less, and more preferably 0.0020% or less. However, since extreme reduction of the N content may lead to an increase in manufacturing costs, it is preferable that the N content be 0.0005% or more.

[0038] Ti: Less than 0.0050% Titanium (Ti) is an element that inevitably contaminates steel and can combine with carbon or nitrogen to form precipitates (carbides, nitrides). When carbides or nitrides are formed, these precipitates themselves degrade the magnetic properties of the non-oriented electrical steel sheet. Furthermore, they inhibit grain growth during finish annealing and stress-relieving annealing, further degrading the magnetic properties of the non-oriented electrical steel sheet. Therefore, the Ti content should be less than 0.0050%. Preferably, the Ti content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the Ti content may lead to increased manufacturing costs, it is preferable that the Ti content be 0.0005% or more.

[0039] Nb: Less than 0.0050% Niobium (Nb) is an element that contributes to increased strength by bonding with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Therefore, the Nb content should be less than 0.0050%. Preferably, the Nb content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the Nb content may lead to increased manufacturing costs, it is preferable that the Nb content be 0.0001% or more.

[0040] Zr: Less than 0.0050% Zr (zirconium) is an element that contributes to increased strength by bonding with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Therefore, the Zr content should be less than 0.0050%. Preferably, the Zr content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the Zr content may lead to increased manufacturing costs, it is preferable that the Zr content be 0.0001% or more.

[0041] V: Less than 0.0050% Vanadium (V) is an element that contributes to increased strength by bonding with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Therefore, the V content should be less than 0.0050%. Preferably, the V content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the V content may lead to increased manufacturing costs, it is preferable that the V content be 0.0001% or more.

[0042] Cu: Less than 0.200% Copper (Cu) is an element that inevitably becomes mixed into steel. Intentionally including Cu increases the manufacturing cost of non-oriented electrical steel sheets. Therefore, in this embodiment, it is not necessary to actively include Cu, and an impurity level is sufficient. The Cu content is set to less than 0.200%, which is the maximum value that can inevitably be mixed in during the manufacturing process. The Cu content is preferably 0.150% or less, and more preferably 0.100% or less. There is no particular lower limit to the Cu content, however, an extreme reduction in Cu content may lead to an increase in manufacturing costs. Therefore, the Cu content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0043] Ni: Less than 0.500% Nickel (Ni) is an element that is inevitably present in steel. However, since Ni is also an element that improves the strength of non-oriented electrical steel sheets, it may be intentionally included. However, because Ni is expensive, the Ni content should be less than 0.500%. Preferably, the Ni content is 0.400% or less, and more preferably 0.300% or less. There is no particular lower limit to the Ni content, but an extreme reduction in Ni content may lead to an increase in manufacturing costs. Therefore, preferably, the Ni content is 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more. Furthermore, if Ni is intentionally included, it is preferable that the Ni content be 0.200% or more.

[0044] Sn: 0.005~0.040% Sn (tin) is a useful element for ensuring low iron loss in non-oriented electrical steel sheets by segregating on the surface of the base material and suppressing oxidation and nitriding during annealing. In addition, Sn also has the effect of improving the texture by segregating at grain boundaries and increasing the magnetic flux density of non-oriented electrical steel sheets. To obtain these effects, the Sn content should be 0.005% or more. Preferably, the Sn content should be 0.010% or more, and more preferably 0.015% or more. On the other hand, if the Sn content is too high, the toughness of the steel decreases, making cold rolling difficult. Therefore, the Sn content should be 0.040% or less. Preferably, the Sn content should be less than 0.040%, more preferably 0.035% or less, and more preferably 0.030% or less.

[0045] Sb: 0~0.040% Antimony (Sb), like tungsten (Sn), is a useful element for ensuring low iron loss in non-oriented electrical steel sheets by segregating on the surface of the base material and suppressing oxidation and nitriding during annealing. Sb also improves the texture by segregating at grain boundaries, thereby increasing the magnetic flux density of the non-oriented electrical steel sheet. Therefore, Sb may be included as needed. However, excessive Sb content can reduce the toughness of the steel, making cold rolling difficult. For this reason, the Sb content should be 0.040% or less. Preferably, the Sb content is 0.030% or less. To reliably obtain the above effects, the Sb content should be 0.005% or more, and more preferably 0.010% or more.

[0046] In the chemical composition of the base material of the non-oriented electrical steel sheet of the present invention, the remainder is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of steel due to raw materials such as ore and scrap, and various factors in the manufacturing process, and which are acceptable within a range that does not adversely affect the present invention.

[0047] Furthermore, there are no specific restrictions on the content of Cr and Mo as impurity elements. In the non-oriented electrical steel sheet and motor core according to this embodiment, the presence of these elements in a range of 0.5% or less each does not particularly affect the properties of the non-oriented electrical steel sheet and motor core according to this embodiment. Similarly, the presence of Ca and Mg in a range of 0.002% or less each does not particularly affect the properties of the non-oriented electrical steel sheet and motor core according to this embodiment. The presence of rare earth elements (REM) in a range of 0.004% or less also does not particularly affect the properties of the non-oriented electrical steel sheet and motor core according to this embodiment. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0048] Although oxygen (O) is also an impurity element, its presence in the range of 0.035% or less does not affect the properties of the non-oriented electrical steel sheet and motor core according to this embodiment. Since oxygen can also be mixed into the steel during the annealing process, its presence in the slab stage (i.e., ladle value) in the range of 0.010% or less does not particularly affect the properties of the non-oriented electrical steel sheet and motor core according to this embodiment.

[0049] In addition to the elements mentioned above, impurity elements such as Pb, Bi, As, B, and Se may be present, but as long as their content is within the range of 0.0050% or less, it will not impair the properties of the non-oriented electrical steel sheet and motor core according to this embodiment.

[0050] The chemical composition of the non-oriented electrical steel sheet and motor core base material according to this embodiment can be determined using various known measurement methods. For example, it can be measured using ICP emission spectrometry, gravimetric analysis, or spark discharge emission spectrometry. In addition, C and S can be measured using combustion-infrared absorption spectrometry, N can be measured using inert gas combustion-thermal conductivity spectrometry, and O can be measured using inert gas fusion-nondispersive infrared absorption spectrometry.

[0051] Furthermore, in the non-oriented electrical steel sheet of this embodiment, from the viewpoint of suppressing nitriding in the surface layer of the steel sheet, the N content [N]s from the surface of the base material to a depth of 20 μm is set to 0.0060% or less. If [N]s is 0.0060% or less, it is possible to suppress iron loss deterioration. [N]s is preferably 0.0055% or less, and more preferably 0.0050% or less.

[0052] On the other hand, in the motor core of this embodiment, further stress-relieving annealing is performed on the non-oriented electrical steel sheet, which leads to further nitriding. However, in order to suppress iron loss degradation, the [N]s in the motor core of this embodiment is set to 0.0070% or less. Preferably, the [N]s of the motor core is 0.0065% or less, and more preferably 0.0060% or less.

[0053] The nitrogen content [N]s of the non-oriented electrical steel sheet and motor core, up to a depth of 20 μm from the surface, is measured by the following procedure. First, the nitrogen content [N]1 of the non-oriented electrical steel sheet or motor core, after removing the insulating coating with a heated alkaline solution, is measured. Next, 20 μm of each surface of the non-oriented electrical steel sheet or motor core is removed by chemical polishing, and the nitrogen content [N]2 of the sample after removal is measured. Then, [N]s is calculated from the measured [N]1, [N]2, and the thickness t (μm) of the non-oriented electrical steel sheet or motor core using the following formula. [N]s=(t×[N]1-(t-40)×[N]2) / 40

[0054] 3.Crystal grain size In the non-oriented electrical steel sheet of this embodiment, the average grain size of the base material is set to 10 to 30 μm. By setting the average grain size of the base material of the non-oriented electrical steel sheet to 10 μm or more, it is possible to minimize the deterioration of hysteresis loss and improve the magnetic properties. On the other hand, by setting the average grain size to 30 μm or less, a significant improvement in the strength of the steel can be obtained. When magnetic properties are important, the average grain size is preferably 15 μm or more, and more preferably 20 μm or more. On the other hand, when strength is important, the average grain size is preferably 25 μm or less, and more preferably 20 μm or less.

[0055] Furthermore, in the motor core of this embodiment, the average grain size of the base material is set to 50 to 120 μm. By setting the average grain size of the base material of the motor core to 50 μm or more, it is possible to suppress the deterioration of hysteresis loss and improve magnetic properties. On the other hand, by setting the average grain size to 120 μm or less, it is possible to suppress the deterioration of iron loss due to increased eddy current loss. The average grain size is preferably 60 μm or more, and more preferably 70 μm or more. In addition, the average grain size is preferably 110 μm or less, and more preferably 100 μm or less.

[0056] In this invention, the average grain size of the non-oriented electrical steel sheet and the base material of the motor core shall be determined in accordance with JIS G 0551:2013 "Microscopic test method for grain size of steel".

[0057] 4. Magnetic properties In the non-oriented electrical steel sheet and motor core according to this embodiment, excellent magnetic properties refer to iron loss W. 10 / 400 This means that the saturation magnetic flux density (Bs) is low and high.

[0058] Here, iron loss W 10 / 400 This refers to the iron loss that occurs under conditions of a maximum magnetic flux density of 1.0 T and a frequency of 400 Hz, and shall be measured in accordance with the Epstein method specified in JIS C 2550-1:2011.

[0059] In the non-oriented electrical steel sheet of this embodiment, iron loss W 10 / 400 The iron loss is 20.0 W / kg or less. On the other hand, in the motor core of this embodiment, iron loss is further reduced by strain-relieving annealing, and the iron loss W 10 / 400 A low W / kg value means 13.5 W / kg or less for plate thicknesses of 0.26 mm or more, 11.5 W / kg or less for plate thicknesses of 0.21 to 0.25 mm, and 10.0 W / kg or less for plate thicknesses of 0.20 mm or less.

[0060] It is not possible to obtain large test specimens, such as Epstein specimens, from a motor core. When evaluating the iron loss of a motor core, the laminated iron core is separated into steel plates, and small single-sheet magnetic test specimens of a size that can be obtained according to the size of the separated steel plates are manufactured by electrical discharge machining. Then, the iron loss value is measured using a small single-sheet tester corresponding to the above-mentioned small test specimens. In this case, the measurement principle shall follow the Single Sheet Tester (SST) method specified in JIS C 2556:2015.

[0061] Furthermore, several types of non-oriented electrical steel sheets are prepared in advance by taking Epstein test specimens and small test specimens as described above, and the iron loss values ​​are measured using the Epstein method and the single-sheet magnetic property measurement method. A conversion formula is then derived from the relationship between the two measured values. The iron loss values ​​measured by the single-sheet tester are then corrected using the above conversion formula to be equivalent to the iron loss values ​​measured by the Epstein method.

[0062] Furthermore, the saturation magnetic flux density Bs is measured using a vibrating sample magnetometer (VSM). In the non-oriented electrical steel sheet according to this embodiment, the saturation magnetic flux density Bs is 1.945T or higher. In the motor core of this embodiment, the saturation magnetic flux density Bs is also 1.945T or higher.

[0063] 5. Mechanical properties The non-oriented electrical steel sheet according to this embodiment has high strength, specifically a tensile strength of 680 MPa or higher. A tensile strength of 690 MPa or higher is preferable, and 700 MPa or higher is more preferable. Here, the tensile strength is measured by performing a tensile test in accordance with JIS Z 2241:2011.

[0064] 6. Plate thickness In the non-oriented electrical steel sheets and the non-oriented electrical steel sheets constituting the motor core according to this embodiment, the sheet thickness is set to 0.10 mm or more from the viewpoint of manufacturing costs for cold rolling and finish annealing. On the other hand, from the viewpoint of reducing iron loss, the sheet thickness is set to 0.30 mm or less. Therefore, the sheet thickness of the non-oriented electrical steel sheets and the non-oriented electrical steel sheets constituting the motor core according to this embodiment is 0.10 to 0.30 mm.

[0065] Furthermore, the effect of nitriding on the surface layer of the steel sheet becomes more pronounced as the sheet thickness decreases. In other words, if nitriding on the surface layer of the steel sheet is not suppressed, the iron loss deteriorates more significantly as the sheet thickness decreases. However, in the present invention, nitriding on the surface layer of the steel sheet is suppressed, so the deterioration of iron loss can be suppressed even when the sheet thickness is thin. Therefore, the effects of the present invention are more pronounced when the sheet thickness is, for example, less than 0.25 mm or 0.20 mm or less.

[0066] 7. Insulating coating In the non-oriented electrical steel sheet and motor core according to this embodiment, it is preferable that the surface of the base material has an insulating coating. Since the non-oriented electrical steel sheet is used after the core blank is punched out and then laminated, providing an insulating coating on the surface of the base material can reduce eddy currents between the sheets, thereby reducing eddy current losses as a core.

[0067] The type of insulating coating is not particularly limited, and known insulating coatings used as insulating coatings for non-oriented electrical steel sheets can be used. Examples of such insulating coatings include composite insulating coatings mainly composed of inorganic materials and further containing organic materials. Here, a composite insulating coating is an insulating coating mainly composed of at least one of the following: metal chromate salts, metal phosphate salts, or inorganic materials such as colloidal silica, Zr compounds, or Ti compounds, with fine organic resin particles dispersed in it. In particular, from the viewpoint of reducing the environmental burden during manufacturing, which has been a growing need in recent years, insulating coatings using metal phosphate salts, Zr or Ti coupling agents, or carbonates or ammonium salts thereof as starting materials are preferably used.

[0068] The amount of insulating coating applied is not particularly limited, but for example, 200-1500 mg / m² per side. 2 It is preferable to use a concentration of approximately 300-1200 mg / m² per side. 2 It is more preferable to do so. By forming the insulating film so that the amount of adhesion falls within the above range, it is possible to maintain excellent uniformity. When measuring the amount of insulating film adhesion afterward, various known measurement methods can be used. For example, a method of measuring the mass difference before and after immersion in a sodium hydroxide aqueous solution, or a fluorescence X-ray method using a calibration curve can be used as appropriate.

[0069] 8. Manufacturing method The non-oriented electrical steel sheet according to this embodiment can be manufactured by sequentially performing a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process, a cold rolling process, and a finish annealing process on a steel ingot having the chemical composition described above, under the conditions shown below. Furthermore, if an insulating film is to be formed on the surface of the base material, an insulating film formation process is performed after the finish annealing process. Each process will be described in detail below.

[0070] <Hot rolling process> A steel ingot (slab) having the above chemical composition is heated, and the heated steel ingot is hot-rolled to obtain a hot-rolled sheet. There is no particular requirement for the heating temperature of the steel ingot before hot-rolling, but it is preferable to set it to, for example, 1050 to 1250°C. There is also no particular requirement for the thickness of the hot-rolled sheet after hot-rolling, but it is preferable to set it to, for example, about 1.5 to 3.0 mm, taking into account the final thickness of the base material.

[0071] <Acid washing process> The hot-rolled sheet described above is subjected to pickling to remove the scale layer formed on the surface of the base material. Here, the pickling conditions, such as the concentration of the acid used, the concentration of the accelerator used, and the temperature of the pickling solution, are not particularly limited and can be known pickling conditions.

[0072] <Batch-type hot-rolled sheet annealing process> Subsequently, hot-rolled sheet annealing is performed with the aim of reducing iron loss in the steel sheet. Hot-rolled sheet annealing is carried out using a batch annealing furnace, with a soaking temperature of 650-780°C and a soaking time of 8-36 hours. By soaking for 8 hours or more, the metal structure is sufficiently homogenized, the coarsening of precipitates progresses, and a sufficient improvement in iron loss can be obtained. On the other hand, if the soaking temperature exceeds 780°C or the soaking time exceeds 36 hours, the grain size becomes excessively coarse, reducing toughness and causing fracture during cold rolling.

[0073] The batch annealing furnace maintains a non-oxidizing atmosphere, which can be a mixed atmosphere of H2 and N2 with an H2 content of 1 to 100 volume percent (i.e., H2 + N2 = 100 volume percent). Even in an atmosphere containing N2, nitriding of the surface layer of the base material can be suppressed if the Mn and Sn content is appropriate. However, from the viewpoint of more reliably suppressing nitriding, an atmosphere with H2:100 volume percent is preferred.

[0074] <Cold rolling process> Cold rolling is performed on the steel sheet after annealing the hot-rolled sheet described above. In cold rolling, the sheet is rolled at a reduction ratio such that the final thickness of the base material is 0.10 to 0.30 mm.

[0075] <Finishing Annealing Process> Following the cold rolling described above, finish annealing is performed. In the method for manufacturing non-oriented electrical steel sheets according to this embodiment, it is preferable to use a continuous annealing furnace for finish annealing. Finish annealing is performed under conditions where the soaking temperature is 700 to 830°C and the soaking time is 1 second to 10 minutes. It is preferable to use a mixed atmosphere of H2 and N2 with an H2 proportion of 1 to 100 volume percent (i.e., H2 + N2 = 100 volume percent) and a dew point of the atmosphere of -50 to +10°C.

[0076] If the soaking temperature is below 700°C, the grain size becomes finer and the amount of unrecrystallized structure increases, resulting in a significant deterioration of iron loss, which is undesirable. If the soaking temperature exceeds 830°C, the strength becomes insufficient, which is also undesirable. Furthermore, if the soaking time is less than 1 second, the dislocation density cannot be sufficiently reduced. On the other hand, if the soaking time exceeds 10 minutes, it leads to an increase in manufacturing costs.

[0077] <Insulating film formation process> Following the above-mentioned finish annealing, an insulating coating formation process is carried out as needed. Here, the method for forming the insulating coating is not particularly limited, and the treatment solution may be applied and dried using a known method using a known insulating coating forming solution as shown below. Examples of known insulating coatings include composite insulating coatings mainly composed of inorganic materials and further containing organic materials.

[0078] A composite insulating coating is an insulating coating that mainly consists of at least one of the following: a metal salt such as a metal chromate salt or a metal phosphate salt, or an inorganic substance such as colloidal silica, a Zr compound, or a Ti compound, with fine organic resin particles dispersed within it. In particular, from the viewpoint of reducing the environmental burden during manufacturing, which has been a growing need in recent years, insulating coatings using a metal phosphate salt, a Zr or Ti coupling agent as a starting material, or insulating coatings using a carbonate or ammonium salt of a metal phosphate salt, a Zr or Ti coupling agent as a starting material are preferably used.

[0079] The surface of the base material on which the insulating coating is formed may be subjected to any pretreatment before applying the treatment solution, such as degreasing with an alkali or pickling with hydrochloric acid, sulfuric acid, or phosphoric acid. Alternatively, the treatment solution may be applied to the surface of the base material directly after finish annealing without any of these pretreatments.

[0080] Furthermore, while the manufacturing method for the motor core according to this embodiment is not particularly limited, it is possible to manufacture it by sequentially performing a punching process, a lamination process, and a stress-relieving annealing process on a non-oriented electrical steel sheet obtained through the above-described process, for example, under the conditions shown below. As mentioned above, it is preferable to perform the stress-relieving annealing described later when the non-oriented electrical steel sheet is used as a stator core where low iron loss is important. Therefore, it is preferable that the motor core according to this embodiment is a stator core. Each process will be described in detail below.

[0081] <Punching process> The non-oriented electrical steel sheet obtained as described above is subjected to punching to form a shape suitable for use as a rotor core or stator core material. There are no particular restrictions on the processing conditions, and general methods can be used.

[0082] <Lamination process> Multiple sheets of non-oriented electrical steel, which have undergone a punching process, are laminated together to form a motor core.

[0083] <Stress Relief Annealing Process> The stacked motor cores are subjected to stress-relieving annealing. Stress-relieving annealing promotes recrystallization and grain growth in the motor cores, reducing iron loss and significantly improving motor efficiency.

[0084] In the motor core manufacturing method according to this embodiment, stress relief annealing is performed under conditions of a soaking temperature of 750 to 900°C and a soaking time of 10 to 180 minutes. If the proportion of N2 in the atmosphere is less than 70 volume%, it will lead to an increase in the cost of stress relief annealing and is therefore undesirable. The proportion of N2 in the atmosphere is more preferably 80 volume% or more, even more preferably 90 to 100 volume%, and particularly preferably 97 to 100 volume%. The atmospheric gas other than N2 is not particularly specified, but generally, a reducing mixed gas consisting of hydrogen, carbon dioxide, carbon monoxide, water vapor, methane, etc. can be used. To obtain these gases, a method of burning propane gas or natural gas is generally employed. The dew point of the atmosphere is preferably -50 to +10°C.

[0085] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0086] Slabs having the chemical composition shown in Table 1 were heated to 1150°C, then hot-rolled at a finishing temperature of 850°C and a finishing thickness of 2.0 mm, and wound at 600°C to obtain hot-rolled steel sheets. After removing scale from the obtained hot-rolled steel sheets by pickling, they were subjected to hot-rolled sheet annealing in a batch annealing furnace at the soaking temperature shown in Table 2 for a soaking time of 10 hours. The resulting steel sheets were then cold-rolled to a thickness of 0.20 mm. Furthermore, finish annealing was performed in a mixed atmosphere of H2:15%, N2:85%, and dew point:-30°C at the soaking temperature shown in Table 2 for a soaking time of 20 seconds. After finish annealing, an insulating coating consisting of aluminum phosphate and an acrylic-styrene copolymer resin emulsion with a particle size of 0.2 μm was applied to the steel sheets and baked in air at 350°C.

[0087] Furthermore, the obtained non-oriented electromagnetic steel sheet was subjected to stress relief annealing under soaking conditions of 800°C for 120 minutes in a nitrogen atmosphere with a dew point of -40°C (the proportion of nitrogen in the atmosphere is 99.9% by volume or more).

[0088]

Table 1

[0089]

Table 2

[0090] For each test material after finish annealing and stress relief annealing, the average crystal grain size was measured on a cross-section parallel to the rolling direction of the base material in accordance with JIS G 0551:2013 "Steel - Microscopic Test Method for Crystal Grain Size". Also, Epstein test pieces were taken from the rolling direction and the width direction of each test material after finish annealing, and the iron loss W 10 / 400 was evaluated by an Epstein test in accordance with JIS C 2550-1:2011. Note that the evaluation of the iron loss W 10 / 400 after stress relief annealing was measured after subjecting the Epstein test pieces taken from each test material after finish annealing to stress relief annealing. The saturation magnetic flux density was measured using a vibrating sample magnetometer (VSM).

[0091] Subsequently, JIS No. 5 tensile test pieces were taken from each test material after finish annealing in accordance with JIS Z 2241:2011 so that the longitudinal direction was in line with the rolling direction of the steel sheet. Then, a tensile test was conducted in accordance with JIS Z 2241:2011 using the above test pieces, and the tensile strength was measured.

[0092] Furthermore, the nitrogen content [N]s of each test material from the surface to a depth of 20 μm after finish annealing and stress-relieving annealing was measured using the following procedure. First, the nitrogen content [N]1 of the test material from which the insulating coating was removed with a heated alkaline solution was measured. Next, 20 μm of each surface of the test material was removed by chemical polishing, and the nitrogen content [N]2 of the sample after removal was measured. Then, [N]s was calculated from the measured [N]1, [N]2, and the plate thickness t (μm) of the test material using the following formula. [N]s=(t×[N]1-(t-40)×[N]2) / 40

[0093] The results mentioned above are also shown in Table 2.

[0094] In tests No. 2, 3, 6, 7, 9, 11, 15-17, 20, 21, 26, 27, 29, and 30, which satisfy the provisions of the present invention, it was found that the iron loss after finish annealing was low, the saturation magnetic flux density was high, the tensile strength was high at 680 MPa or more, and the iron loss after strain relief annealing was also low.

[0095] In contrast to these, in comparative studies No. 1, 4, 5, 8, 10, 12-14, 18, 19, 22-25, 28, 31 and 32, the iron loss W 10 / 400 Either the material is inferior, the saturation magnetic flux density is poor, or the toughness deteriorates significantly, making manufacturing difficult.

[0096] Specifically, in Test No. 1, the Mn content was lower than the specified range, resulting in a large amount of fine MnS precipitation and poor iron loss. In Test No. 4, the S content was higher than the specified range, resulting in a large amount of MnS precipitation and poor iron loss. In Test No. 5, the Sn content was lower than the specified range, resulting in a high [N]s content and poor iron loss. In Test No. 8, the Sn content was higher than the specified range, causing toughness to deteriorate and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0097] In Test No. 10, the Mn content was higher than the specified range, resulting in a high [N]s value and inferior iron loss. In Test No. 12, the Si + Al + 0.5 × Mn content was higher than the specified range, resulting in inferior saturation magnetic flux density. In Test No. 13, the Si content and Si + Al + 0.5 × Mn content were higher than the specified range, leading to degraded toughness and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0098] In Test No. 14, the hot-rolled sheet annealing temperature was higher than the specified range, resulting in degraded toughness and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties. In Test No. 18, the hot-rolled sheet annealing temperature was lower than the specified range, resulting in smaller average grain sizes after finish annealing and stress-relieving annealing, and consequently, lower iron loss after finish annealing and stress-relieving annealing. In Test No. 19, the finish annealing temperature was lower than the specified range, resulting in smaller average grain sizes after finish annealing, and consequently, lower iron loss after finish annealing. In Test No. 22, the finish annealing temperature was higher than the specified range, resulting in lower tensile strength.

[0099] In Test No. 23, the Si content was lower than the specified range, resulting in inferior tensile strength. In Test No. 24, the Si + Al + 0.5 × Mn content was lower than the specified range, resulting in inferior iron loss after finish annealing and stress-relieving annealing. Furthermore, in Test No. 25, the Al content was lower than the specified range, leading to fine AlN precipitation and inferior iron loss after finish annealing, as well as a small average grain size after stress-relieving annealing, resulting in inferior iron loss after stress-relieving annealing. Finally, in Test No. 28, the Al content was higher than the specified range, resulting in degraded toughness and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0100] In Test No. 31, the soaking temperature during hot-rolled sheet annealing was higher than the specified range, resulting in grain coarsening and degraded toughness, causing fracture during cold rolling, and making it impossible to measure tensile strength and magnetic properties. In Test No. 32, the Al content was lower than the specified range, resulting in a degraded texture and precipitation of fine AlN particles, leading to poor iron loss after finish annealing and poor iron loss after stress-relieving annealing. [Industrial applicability]

[0101] As described above, according to the present invention, non-oriented electrical steel sheets with high strength and excellent magnetic properties can be obtained at low cost.

Claims

1. The chemical composition of the base material is, in mass percent, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: Less than 0.200% Ni: Less than 0.500%, Sn: 0.005-0.040%, Sb: 0 to 0.040%, The remainder consists of Fe and impurities. The following equation (i) is satisfied, The N content [N]s from the surface of the base material to a depth of 20 μm is 0.0060% or less. The average grain size of the base material is 10 to 30 μm. Iron loss W １０／４００ It is 20.0 W / kg or less, The saturation magnetic flux density is 1.945 T or higher. The tensile strength is 680 MPa or more. The plate thickness is 0.10 to 0.30 mm. Non-oriented electrical steel sheet. 4.2≦Si+Al+0.5×Mn≦4.9...(i) However, the element symbols in the above formula represent the content (mass %) of each element.

2. The surface of the base material has an insulating coating, The non-oriented electrical steel sheet according to claim 1.

3. A method for manufacturing a non-oriented electrical steel sheet according to claim 1 or claim 2, In mass percent, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: Less than 0.200% Ni: Less than 0.500%, Sn: 0.005-0.040%, Sb: 0 to 0.040%, The remainder consists of Fe and impurities. For a steel ingot having a chemical composition satisfying the following equation (i), The process involves, in order, a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process at a soaking temperature of 650 to 780°C for 8 to 36 hours, a cold rolling process to reduce the sheet thickness to 0.10 to 0.30 mm, and a finish annealing process at a soaking temperature of 700 to 830°C for 1 second to 10 minutes. A method for manufacturing non-oriented electrical steel sheets. 4.2≦Si+Al+0.5×Mn≦4.9...(i) However, the element symbols in the above formula represent the content (mass %) of each element.

4. A motor core in which multiple non-oriented electrical steel sheets are laminated, The chemical composition of the base material of the aforementioned non-oriented electrical steel sheet is, in mass%, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: Less than 0.200% Ni: Less than 0.500%, Sn: 0.005-0.040%, Sb: 0 to 0.040%, The remainder consists of Fe and impurities. The following equation (i) is satisfied, The N content [N]s from the surface of the base material to a depth of 20 μm is 0.0070% or less. The average grain size of the base material is 50 to 120 μm. The saturation magnetic flux density is 1.945 T or higher. The thickness of the aforementioned non-oriented electrical steel sheet is 0.10 to 0.30 mm. Motor core. 4.2≦Si+Al+0.5×Mn≦4.9...(i) However, the element symbols in the above formula represent the content (mass %) of each element.

5. The surface of the base material has an insulating coating, The motor core according to claim 4.

6. A method for manufacturing a motor core according to claim 4 or claim 5, In mass percent, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: Less than 0.200% Ni: Less than 0.500%, Sn: 0.005-0.040%, Sb: 0 to 0.040%, The remainder consists of Fe and impurities. For a steel ingot having a chemical composition satisfying the following equation (i), The process involves, in order, a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process at a soaking temperature of 650-780°C for 8-36 hours, a cold rolling process to reduce the sheet thickness to 0.10-0.30 mm, a finish annealing process at a soaking temperature of 700-830°C for 1 second to 10 minutes, a punching process, a lamination process, and a stress-relieving annealing process at a soaking temperature of 750-900°C for 10-180 minutes. A method for manufacturing a motor core. 4.2≦Si+Al+0.5×Mn≦4.9...(i) However, the element symbols in the above formula represent the content (mass %) of each element.

Images