Non-oriented electrical steel sheet, method for producing same, and motor core

Abstract

Provided are: a high-strength non-oriented electrical steel sheet having good fatigue characteristics suitable for use in a rotor core; and a non-oriented electrical steel sheet having excellent magnetic characteristics suitable for use in a stator core. The non-oriented electrical steel sheet has a component composition containing, in mass%, 0.01% or less of C, 2.0% or more and less than 4.5% of Si, 0.05%-5.00% of Mn, 0.1% or less of P, 0.01% or less of S, 3.0% or less of Al, 0.005% or less of N, less than 4.5% of Si + Al, and the balance of Fe and unavoidable impurities, the average crystal grain size X1 of crystal grains in the steel sheet being 50 [mu] m or less, and the average crystal grain size X2 of crystal grains in the steel sheet being 50 [mu] m or less. The standard deviation S1 of the crystal particle size distribution satisfies S1 / X1 < 0.75, and the sharpness K1 of the crystal particle size distribution is 20.0 or less.

Description

Technical Field

[0001] The present invention relates to a non-oriented electromagnetic steel sheet, a method for manufacturing the same, and a motor core using the non-oriented electromagnetic steel sheet. Background Art

[0002] In recent years, the demand for energy saving of electronic devices has been increasing worldwide. Along with this, there has also been a demand for more excellent magnetic properties for non-oriented electromagnetic steel sheets used for cores of rotating machines. In addition, recently, in drive motors of HEVs (hybrid electric vehicles) and EVs (electric vehicles), there has been a strong demand for miniaturization and high output, and in order to meet this demand, research is being conducted to increase the rotational speed of motors.

[0003] A motor core is divided into a stator core and a rotor core. The rotor core of an HEV drive motor generates a large centrifugal force due to its large outer diameter. In addition, there is a very narrow portion (width: 1 to 2 mm) called a rotor core bridging portion in the structure of the rotor core, and this portion becomes a state where the stress is particularly high during motor driving. Furthermore, due to the repeated rotation and stop of the motor, a large repeated stress caused by the centrifugal force acts on the rotor core. Therefore, the electromagnetic steel sheet used for the rotor core needs to have excellent fatigue characteristics.

[0004] On the other hand, in order to achieve miniaturization and high output of the motor, the electromagnetic steel sheet used for the stator core is preferably a high magnetic flux density and low iron loss. That is, as the characteristics required for the electromagnetic steel sheet used for the motor core, it is ideal that the electromagnetic steel sheet for the rotor core has excellent fatigue characteristics, and the electromagnetic steel sheet for the stator core has a high magnetic flux density and low iron loss.

[0005] Thus, even for electromagnetic steel sheets used for the same motor core, the characteristics required for the rotor core and the stator core are very different. However, in the manufacture of a motor core, in order to improve the material yield and productivity, it is preferable to take the rotor core material and the stator core material from the same base material steel sheet by blanking, and then stack the respective steel sheets to assemble the rotor core or the stator core.

[0006] As a technique for manufacturing a high-strength and low-iron-loss non-oriented electromagnetic steel sheet for a motor core, for example, Patent Document 1 discloses the following technique for manufacturing a high-strength rotor core and a low-iron-loss stator core from the same base material: manufacturing a high-strength non-oriented electromagnetic steel sheet, taking the rotor core material and the stator core material from this steel sheet by blanking and stacking them, assembling the rotor core and the stator core, and then performing stress relief annealing only on the stator core.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2008-50686 Summary of the Invention

[0010] However, according to the research by the present inventors, in the technology disclosed in the above Patent Document 1, by using a high-strength non-oriented electrical steel sheet to increase the yield stress, there is a concern that the fatigue strength, which is the most important characteristic, may not necessarily be improved. Also, in the technology disclosed in Patent Document 1, there is a problem that the iron loss value after stress relief annealing may not be stably achieved at the level required industrially.

[0011] The present invention has been completed in view of the problems of the above prior art, and its object is to provide a high-strength non-oriented electrical steel sheet having good fatigue characteristics suitable for a rotor core and a non-oriented electrical steel sheet having excellent magnetic characteristics suitable for a stator core, and to propose a method for manufacturing the non-oriented electrical steel sheet at low cost.

[0012] The present inventors have conducted in-depth research to solve the above problems, and as a result, it has been found that by controlling the crystal grain size distribution, a non-oriented electrical steel sheet with high fatigue strength can be obtained, and when the non-oriented electrical steel sheet is subjected to grain growth by stress relief annealing (heat treatment), excellent low iron loss can be stably achieved. Also, it has been found that by optimizing the rolling conditions of the final pass of cold rolling, the crystal grain size distribution can be controlled.

[0013] The present invention has been completed based on this insight and has the following configuration.

[0014] [1] A non-oriented electrical steel sheet, characterized in that it has the following composition:

[0015] Containing C: 0.01% or less, Si: 2.0% or more and less than 4.5%, Mn: 0.05% to 5.00%, P: 0.1% or less, S: 0.01% or less, Al: 3.0% or less, and N: 0.0050% or less by mass%, Si + Al is less than 4.5%, and the balance is Fe and inevitable impurities.

[0016] For the crystal grains in the steel sheet, the average crystal grain size X 1 is 50 μm or less, the standard deviation S of the crystal grain size distribution 1 satisfies the following formula (1), and the kurtosis K of the crystal grain size distribution 1 is 20.0 or less.

[0017] S 1 / X 1 <0.75…(1)

[0018] [2]The non-oriented electromagnetic steel sheet according to [1] above, wherein the above composition further contains Co: 0.0005% to 0.0050% by mass.

[0019] [3]The non-oriented electromagnetic steel sheet according to [1] or [2] above, wherein the above composition further contains Cr: 0.05% to 5.00% by mass.

[0020] [4]The non-oriented electromagnetic steel sheet according to any one of [1] to [3] above, wherein the above composition further contains any one or more of Ca: 0.001% to 0.100%, Mg: 0.001% to 0.100%, and REM: 0.001% to 0.100% by mass.

[0021] [5]The non-oriented electromagnetic steel sheet according to any one of [1] to [4] above, wherein the above composition further contains any one or both of Sn: 0.001% to 0.200% and Sb: 0.001% to 0.200% by mass.

[0022] [6]The non-oriented electromagnetic steel sheet according to any one of [1] to [5] above, wherein the above composition further contains any one or more of Cu: 0% to 0.5%, Ni: 0% to 0.5%, Ti: 0% to 0.005%, Nb: 0% to 0.005%, V: 0% to 0.010%, Ta: 0% to 0.002%, B: 0% to 0.002%, Ga: 0% to 0.005%, Pb: 0% to 0.002%, Zn: 0% to 0.005%, Mo: 0% to 0.05%, W: 0% to 0.05%, Ge: 0% to 0.05%, and As: 0% to 0.05% by mass.

[0023] [7]A non-oriented electromagnetic steel sheet, characterized in that

[0024] it has the composition according to any one of [1] to [6] above,

[0025] For the crystal grains in the steel sheet, the average crystal grain diameter X 2 is 80 μm or more, and the standard deviation S of the crystal grain diameter distribution 2 satisfies the following formula (2), and the kurtosis K of the crystal grain diameter distribution 2 is 3.00 or less.

[0026] S 2 / X 2 <0.75…(2)

[0027] [8]A method for manufacturing a non-oriented electromagnetic steel sheet, which is a method for manufacturing the non-oriented electromagnetic steel sheet described in any one of [1] to [6] above, comprises the following steps:

[0028] A hot rolling step of performing hot rolling on a steel billet having the composition described in any one of [1] to [6] above to obtain a hot rolled sheet;

[0029] A pickling step of pickling the above hot rolled sheet;

[0030] A cold rolling step of cold rolling the hot rolled sheet subjected to the above pickling under the conditions that the working roll diameter D in the final pass is 150 mmφ or more, the reduction ratio r in the final pass is 15% or more, and the strain rate ε m is 100 s -1 to 1300 s -1 to obtain a cold rolled sheet; and

[0031] An annealing step of heating the above cold rolled sheet to an annealing temperature T of 700°C to 850°C under the condition that the average heating rate V from 500°C to 700°C 1 is 10°C / s or more, and then cooling to obtain a cold rolled annealed sheet as the non-oriented electromagnetic steel sheet. 2

[0032] [9]A method for manufacturing a non-oriented electromagnetic steel sheet, which is a method for manufacturing the non-oriented electromagnetic steel sheet described in [7] above, comprises a heat treatment step of heating the non-oriented electromagnetic steel sheet described in any one of [1] to [6] above to a heat treatment temperature T of 750°C to 900°C 3 .

[0033]

[10] A motor core is composed of a rotor core and a stator core. The rotor core is a laminate of the non-oriented electromagnetic steel sheets described in any one of [1] to [6] above, and the stator core is a laminate of the non-oriented electromagnetic steel sheets described in [7] above.

[0034] According to the present invention, a non-oriented electromagnetic steel sheet having good fatigue strength suitable for a rotor core and a non-oriented electromagnetic steel sheet having excellent magnetic properties (low iron loss) suitable for a stator core can be provided. Moreover, these non-oriented electromagnetic steel sheets can be provided by the same steel sheet. Therefore, by using the non-oriented electromagnetic steel sheet of the present invention, a high-performance motor core can be provided at low cost with a good material yield. The non-oriented electromagnetic steel sheet of the present invention is also applicable to a small and high-output motor. Detailed Description

[0035] The details of the present invention and the reasons for the limitations will be described below.

[0036] <Composition of the non-oriented electrical steel sheet>

[0037] The preferred composition of the non-oriented electrical steel sheet and the motor core of the present invention will be described. The unit of the element content in the composition is "mass%", and hereinafter, unless otherwise specified, it is only expressed as "%".

[0038] It should be noted that as the non-oriented electrical steel sheet of the present invention, a first non-oriented electrical steel sheet mainly applicable to the rotor core and a second non-oriented electrical steel sheet mainly applicable to the stator core can be cited. However, since these non-oriented electrical steel sheets are obtained from the same steel sheet, the preferred composition is the same in the first non-oriented electrical steel sheet and the second non-oriented electrical steel sheet.

[0039] C: 0.01% or less

[0040] C is a harmful element that forms carbides during the use of the motor to cause magnetic aging and deteriorate the iron loss characteristics. In order to avoid magnetic aging, the C content in the steel sheet is 0.01% or less. The preferred C content is 0.004% or less. It should be noted that the lower limit of the C content is not particularly specified, but a steel sheet with an excessively reduced C is very expensive, so the C content is preferably 0.0001% or more.

[0041] Si: 2.0% or more and less than 4.5%

[0042] Si has the effect of increasing the inherent resistance of the steel and reducing iron loss, and also has the effect of increasing the strength of the steel by solid solution strengthening. In order to obtain such an effect, the Si content is 2.0% or more. On the other hand, if the Si content is 4.5% or more, the magnetic flux density decreases significantly with the decrease of the saturation magnetic flux density, so the Si content is less than 4.5%. Therefore, the Si content is in the range of 2.0% or more and less than 4.5%. The Si content is preferably 2.5% or more and less than 4.5%, and more preferably 3.0% or more and less than 4.5%.

[0043] Mn: 0.05% - 5.00%

[0044] Mn, like Si, is an element useful for increasing the inherent resistance and strength of the steel. In order to obtain such an effect, the Mn content needs to be 0.05% or more. On the other hand, if the Mn content exceeds 5.00%, the precipitation of MnC may be promoted and the magnetic properties may deteriorate, so the upper limit of the Mn content is 5.00%. Therefore, the Mn content is 0.05% - 5.00%. The Mn content is preferably 0.10% or more, and preferably 3.00% or less.

[0045] P: 0.1% or less

[0046] P is a useful element for adjusting the strength (hardness) of steel. However, if the P content exceeds 0.1%, the toughness decreases and cracks are likely to occur during processing. Therefore, the P content is set below 0.1%. It should be noted that the lower limit of the P content is not particularly specified, but steel plates with an overly reduced P content are very expensive. Therefore, the P content is preferably 0.001% or more. The P content is preferably 0.003% or more and is preferably 0.08% or less.

[0047] S: 0.01% or less

[0048] S is an element that forms fine precipitates and has an adverse effect on the iron loss characteristics. In particular, if the S content exceeds 0.01%, its adverse effect becomes significant. Therefore, the S content is set below 0.01%. It should be noted that the lower limit of the S content is not particularly specified, but steel plates with an overly reduced S content are very expensive. Therefore, the S content is preferably 0.0001% or more. The S content is preferably 0.0003% or more and is preferably 0.0080% or less, more preferably 0.0050% or less.

[0049] Al: 3.0% or less

[0050] Al, like Si, is a useful element that has the effect of increasing the specific resistance of steel and reducing iron loss. To obtain such an effect, the Al content is preferably 0.005% or more. The Al content is more preferably 0.010% or more and further preferably 0.015% or more. On the other hand, if the Al content exceeds 3.0%, it sometimes promotes nitriding on the steel plate surface and deteriorates the magnetic properties. Therefore, the upper limit of the Al content is 3.0%. The Al content is preferably 2.0% or less.

[0051] N: 0.0050% or less

[0052] N is an element that forms fine precipitates and has an adverse effect on the iron loss characteristics. In particular, if the N content exceeds 0.0050%, its adverse effect becomes significant. Therefore, the N content is set below 0.0050%. The N content is preferably 0.0030% or less. It should be noted that the lower limit of the N content is not particularly specified, but steel plates with an overly reduced N content are very expensive. Therefore, the N content is preferably 0.0005% or more. The N content is preferably 0.0008% or more and is preferably 0.0030% or less.

[0053] Si + Al: less than 4.5%

[0054] By making Si + Al (the total content of Si and Al) less than 4.5% and further performing cold rolling under appropriate conditions, an effect of reducing the sharpness of the crystal grain size distribution of the cold-rolled annealed sheet can be obtained. Thereby, the fatigue strength increases, and when grain growth is achieved by stress relief annealing (heat treatment), excellent low iron loss characteristics can be expected. Therefore, the value of Si + Al is made less than 4.5%. It should be noted that the reason for reducing the sharpness of the crystal grain size distribution by making the value of Si + Al less than 4.5% and combining appropriate cold rolling is not yet clear. However, in this regard, the inventors et al. speculate that this is due to the optimization of the shear strain distribution during cold rolling due to the change in the balance of the slip systems active during cold rolling.

[0055] In the composition of the electromagnetic steel sheet of one embodiment, the remaining part other than the above components is Fe and inevitable impurities. However, the composition of the electromagnetic steel sheet of other embodiments may further contain one or more selected from the elements described below in a specified amount based on the above components (elements) according to the required characteristics.

[0056] Co: 0.0005% - 0.0050%

[0057] Co has the effect of strengthening the effect of reducing the sharpness of the crystal grain size distribution of the annealed sheet by appropriate control of Si + Al and cold rolling conditions. That is, by adding a small amount of Co, the sharpness of the crystal grain size distribution can be stably reduced. To obtain such an effect, the Co content can be made 0.0005% or more. On the other hand, if the Co content exceeds 0.0050%, the effect saturates, which instead leads to an increase in cost. Therefore, when adding Co, the upper limit of the Co content is made 0.0050%. Therefore, the above composition preferably further contains Co: 0.0005% - 0.0050%.

[0058] Cr: 0.05% - 5.00%

[0059] Cr has the effect of increasing the inherent resistance of the steel and reducing iron loss. To obtain such an effect, the Cr content can be made 0.05% or more. On the other hand, if the Cr content exceeds 5.00%, the magnetic flux density decreases significantly along with the decrease in the saturation magnetic flux density. Therefore, when adding Cr, the upper limit of the Cr content is made 5.00%. Therefore, the above composition preferably further contains Cr: 0.05% - 5.00%.

[0060] Ca: 0.001% - 0.100%

[0061] Ca is an element that helps reduce iron loss by fixing S as sulfide. To achieve such an effect, the Ca content can be 0.001% or more. On the other hand, if the Ca content exceeds 0.100%, the effect saturates and instead causes an increase in cost. Therefore, when adding Ca, the upper limit of the Ca content is 0.100%.

[0062] Mg: 0.001% - 0.100%

[0063] Mg is an element that helps reduce iron loss by fixing S as sulfide. To achieve such an effect, the Mg content can be 0.001% or more. On the other hand, if the Mg content exceeds 0.100%, the effect saturates and instead causes an increase in cost. Therefore, when adding Mg, the upper limit of the Mg content is 0.100%.

[0064] REM: 0.001% - 0.100%

[0065] REM is a group of elements that help reduce iron loss by fixing S as sulfide. To achieve such an effect, the REM content can be 0.001% or more. On the other hand, if the REM content exceeds 0.100%, the effect saturates and instead causes an increase in cost. Therefore, when adding REM, the upper limit of the REM content is 0.100%.

[0066] From the same perspective, the above component composition preferably further contains any one or two or more of Ca: 0.001% - 0.100%, Mg: 0.001% - 0.100%, and REM: 0.001% - 0.100%.

[0067] Sn: 0.001% - 0.200%

[0068] Sn is an effective element that improves the magnetic flux density and reduces iron loss by improving the aggregation structure. To achieve such an effect, the Sn content can be 0.001% or more. On the other hand, if the Sn content exceeds 0.200%, the effect saturates and instead causes an increase in cost. Therefore, when adding Sn, the upper limit of the Sn content is 0.200%.

[0069] Sb: 0.001% - 0.200%

[0070] Sb is an effective element that improves the magnetic flux density and reduces iron loss by improving the aggregation structure. To achieve such an effect, the Sb content can be 0.001% or more. On the other hand, if the Sb content exceeds 0.200%, the effect saturates and instead causes an increase in cost. Therefore, when adding Sb, the upper limit of the Sb content is 0.200%.

[0071] From the same perspective, the above composition preferably further contains any one or two of Sn: 0.001% to 0.200% and Sb: 0.001% to 0.200%.

[0072] Cu: 0% to 0.5%

[0073] Cu is an element that improves the toughness of steel and can be added appropriately. However, if the content of Cu exceeds 0.5%, the effect saturates. Therefore, when adding Cu, the upper limit of the Cu content is set to 0.5%. When adding Cu, the Cu content is more preferably 0.01% or more and more preferably 0.1% or less. It should be noted that the Cu content can be 0%.

[0074] Ni: 0% to 0.5%

[0075] Ni is an element that improves the toughness of steel and can be added appropriately. However, if the content of Ni exceeds 0.5%, the effect saturates. Therefore, when adding Ni, the upper limit of the Ni content is set to 0.5%. When adding Ni, the Ni content is more preferably 0.01% or more and more preferably 0.1% or less. It should be noted that the Ni content can be 0%.

[0076] Ti: 0% to 0.005%

[0077] Ti forms fine carbonitrides and improves the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of Ti exceeds 0.005%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding Ti, the upper limit of the Ti content is set to 0.005%. The Ti content is more preferably 0.002% or less. It should be noted that the Ti content can be 0%.

[0078] Nb: 0% to 0.005%

[0079] Nb forms fine carbonitrides and improves the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of Nb exceeds 0.005%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding Nb, the upper limit of the Nb content is set to 0.005%. The Nb content is more preferably 0.002% or less. It should be noted that the Nb content can be 0%.

[0080] V: 0% to 0.010%

[0081] V forms fine carbonitrides and enhances the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of V exceeds 0.010%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding V, the upper limit of the V content is set at 0.010%. The V content is more preferably 0.005% or less. It should be noted that the V content can be 0%.

[0082] Ta: 0% to 0.002%

[0083] Ta forms fine carbonitrides and enhances the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of Ta exceeds 0.002%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding Ta, the upper limit of the Ta content is set at 0.0020%. The Ta content is more preferably 0.001% or less. It should be noted that the Ta content can be 0%.

[0084] B: 0% to 0.002%

[0085] B forms fine nitrides and enhances the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of B exceeds 0.002%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding B, the upper limit of the B content is set at 0.002%. The B content is more preferably 0.001% or less. It should be noted that the B content can be 0%.

[0086] Ga: 0% to 0.005%

[0087] Ga forms fine nitrides and enhances the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of Ga exceeds 0.005%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding Ga, the upper limit of the Ga content is set at 0.005%. The Ga content is more preferably 0.002% or less. It should be noted that the Ga content can be 0%.

[0088] Pb: 0% to 0.002%

[0089] Pb forms fine Pb particles and enhances the strength of the steel sheet through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of Pb exceeds 0.002%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when adding Pb, the upper limit of the Pb content is set at 0.002%. The Pb content is more preferably 0.001% or less. It should be noted that the Pb content can be 0%.

[0090] Zn: 0% to 0.005%

[0091] Zn is an element that increases fine inclusions and iron loss. In particular, if the content exceeds 0.005%, the adverse effects become significant. Therefore, when Zn is added, the upper limit of the Zn content is set to 0.005%. The Zn content is more preferably 0.003% or less. It should be noted that the Zn content can be 0%.

[0092] Mo: 0% to 0.05%

[0093] Mo forms fine carbides and improves the steel plate strength through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of Mo exceeds 0.05%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when Mo is added, the upper limit of the Mo content is set to 0.05%. The Mo content is more preferably 0.02% or less. It should be noted that the Mo content can be 0%.

[0094] W: 0% to 0.05%

[0095] W forms fine carbides and improves the steel plate strength through precipitation strengthening, thereby improving the fatigue strength. Therefore, it can be added appropriately. On the other hand, if the content of W exceeds 0.05%, the grain growth property in the heat treatment process deteriorates, resulting in an increase in iron loss. Therefore, when W is added, the upper limit of the W content is set to 0.05%. The W content is more preferably 0.02% or less. It should be noted that the W content can be 0%.

[0096] Ge: 0% to 0.05%

[0097] Ge is an effective element that improves the magnetic flux density and reduces iron loss by improving the microstructure. Therefore, it can be added appropriately. On the other hand, if the content of Ge exceeds 0.05%, the effect saturates. Therefore, when Ge is added, the upper limit of the Ge content is set to 0.05%. The Ge content is more preferably 0.002% or more and more preferably 0.01% or less. It should be noted that the Ge content can be 0%.

[0098] As: 0% to 0.05%

[0099] As is an effective element that improves the magnetic flux density and reduces iron loss by improving the microstructure. Therefore, it can be added appropriately. On the other hand, if the content of As exceeds 0.05%, the effect saturates. Therefore, when As is added, the upper limit of the As content is set to 0.05%. The As content is more preferably 0.002% or more and more preferably 0.01% or less. It should be noted that the As content can be 0%.

[0100] In the above composition, the remaining part other than the above components is Fe and inevitable impurities.

[0101] <Microstructure of the first non-oriented electrical steel sheet>

[0102] Next, the microstructure (grain morphology) in the first non-oriented electrical steel sheet of the present invention will be described. This first non-oriented electrical steel sheet is particularly suitable as a material for a rotor core.

[0103] (Average crystal grain diameter X 1 : 50 μm or less)

[0104] According to the research by the inventors etc., it has been clarified that since the crystal grains in the steel sheet are fine, the fatigue strength is improved. That is, if the average crystal grain diameter X 1 is 50 μm or less, the fatigue strength can meet the value required for the material for the rotor of a motor for HEV or EV (hereinafter referred to as HEV / EV motor). Therefore, in the first non-oriented electrical steel sheet, the average crystal grain diameter X 1 is made 50 μm or less. Here, for the fatigue strength, the value required for the material for the rotor is 500 MPa or more. On the other hand, there is no particular regulation for the lower limit of the average crystal grain diameter X 1 , but if the crystal grain diameter is excessively fine, the ductility of the steel sheet decreases and processing becomes difficult. Therefore, the average crystal grain diameter X 1 is preferably 1 μm or more.

[0105] (Standard deviation S of the crystal grain diameter distribution 1 : Satisfies formula (1))

[0106] When the value of the standard deviation of the crystal grain diameter distribution is larger than the average crystal grain diameter, it promotes stress concentration during repeated stress loading, and thus the fatigue strength decreases. Therefore, in the first non-oriented electrical steel sheet, in order to make the fatigue limit meet the above value required for the rotor material of the HEV / EV motor, the standard deviation S of the crystal grain diameter distribution 1 satisfies the following formula (1):

[0107] S 1 / X 1 <0.75…(1).

[0108] In addition, the standard deviation S of the crystal grain diameter distribution of the first non-oriented electrical steel sheet 1 preferably satisfies the following formula (1'):

[0109] S 1 / X 1 <0.70…(1').

[0110] (Kurtosis K of the crystal grain diameter distribution1 : 20.0 or less)

[0111] The inventors found that by controlling the sharpness of the crystal grain size distribution, an non-oriented electrical steel sheet with excellent fatigue strength can be obtained, and excellent low iron loss can be achieved when the grains grow by stress relief annealing (heat treatment). By simultaneously controlling the sharpness of the crystal grain size distribution and the standard deviation S of the above crystal grain size distribution 1 , such an effect can be obtained.

[0112] Here, the sharpness corresponds to the (specimen) kurtosis in JIS Z8101-1:2015 and is related to the weight of the edge part of the distribution. JIS Z8101-1:2015 corresponds to ISO 3534-1:2006. When the sharpness is high, it means that even for a distribution with the same standard deviation, compared with the case where the distribution shape is a normal distribution, there is a high probability of the existence of values that deviate extremely from the average. That is, in this specification, the sharpness is an index of the frequency of the existence of extremely large grains and / or extremely fine grains with respect to the deviation of the crystal grain size distribution. When the sharpness is high, the existence frequency of extremely large grains and / or extremely fine grains is high. If extremely large grains or extremely fine grains are mixed, excessive stress concentration and resulting local repeated strain are likely to occur during repeated stress loading, so the fatigue characteristics deteriorate. Specifically, if the sharpness K of the crystal grain size distribution 1 is 20.0 or less, the existence frequency of extremely large grains or extremely fine grains is sufficiently small, the blanking fatigue limit satisfies the above value required for the rotor material of HEV / EV motors, and low iron loss after stress relief annealing can be achieved. Therefore, in the first non-oriented electrical steel sheet, the sharpness K of the crystal grain size distribution 1 is 20.0 or less. The sharpness K of the crystal grain size distribution in the first non-oriented electrical steel sheet 1 is preferably 15.0 or less. It should be noted that the lower limit of the above sharpness K 1 does not need to be particularly limited, but it is usually 0 or more even in the case of manufacturing by the method of the present invention.

[0113] It should be noted that the sharpness K 1 can be obtained according to the steps described in the following examples, and is a value calculated using a formula that adjusts the value of the normal distribution to 0.

[0114] <Microstructure of the second non-oriented electrical steel sheet>

[0115] The first non-oriented electrical steel sheet having the above-described microstructure (grain morphology) can become a second non-oriented electrical steel sheet when heat treatment is performed to grow the grains as described later. Therefore, next, the microstructure (grain morphology) in the second non-oriented electrical steel sheet of the present invention will be described. This second non-oriented electrical steel sheet is a non-oriented electrical steel sheet particularly suitable for a stator core.

[0116] (Average crystal grain diameter X 2 : 80 μm or more)

[0117] The iron loss of the non-oriented electrical steel sheet varies according to the average crystal grain diameter. In the second non-oriented electrical steel sheet suitable for a stator core, the average crystal grain diameter X 2 is 80 μm or more. Thereby, the target iron loss characteristics (W 10 / 400 ≤ 13.0 (W / kg)) can be achieved.

[0118] (Standard deviation S of crystal grain diameter distribution 2 : Satisfies formula (2))

[0119] When the value of the standard deviation of the crystal grain diameter distribution is larger than the average crystal grain diameter, there are many excessively fine grains or excessively coarse grains that are disadvantageous for reducing the iron loss, and thus the iron loss increases. Therefore, in the second non-oriented electrical steel sheet, in order to make the iron loss show the above-described target value required for the stator material of the HEV / EV motor, the standard deviation S of the crystal grain diameter distribution 2 satisfies the following formula (2):

[0120] S 2 / X 2 <0.75…(2).

[0121] In addition, the standard deviation S of the crystal grain diameter distribution of the second non-oriented electrical steel sheet 2 preferably satisfies the following formula (2'):

[0122] S 2 / X 2 <0.70…(2').

[0123] (Kurtosis K of crystal grain diameter distribution 2 : 3.00 or less)

[0124] The inventors of the present invention have found that excellent low iron loss can be achieved by controlling the kurtosis of the crystal grain diameter distribution. By simultaneously controlling the kurtosis of the crystal grain diameter and the above-described standard deviation S of the crystal grain diameter distribution 2, such an effect can be obtained. As described above, in the case of high sharpness in this specification, the frequency of existence of extremely coarse grains and / or extremely fine grains is high. The extremely coarse grains or extremely fine grains induce an increase in eddy current loss and deteriorate the iron loss characteristics of the entire steel sheet. Specifically, if the sharpness K 2 of the crystal grain size distribution is 3.00 or less, the frequency of existence of extremely coarse grains or extremely fine grains is sufficiently small, and the iron loss shows a good value required for the stator material of HEV / EV motors. Therefore, in the second non-oriented electrical steel sheet, the sharpness K 2 of the crystal grain size distribution is 3.00 or less. The sharpness K 2 of the crystal grain size distribution in the second non-oriented electrical steel sheet is preferably 2.50 or less, more preferably 2.00 or less. On the other hand, the lower limit of the above-mentioned sharpness K 2 does not need to be particularly specified, but it is usually 0 or more even in the case of manufacturing by the method of the present invention.

[0125] It should be noted that the sharpness K 2 can be obtained according to the steps described in the following examples and is a value calculated using a formula that adjusts the value of the normal distribution to 0.

[0126] <Motor core>

[0127] The motor core of the present invention is composed of a rotor core and a stator core. The rotor core is the above-mentioned first non-oriented electrical steel sheet, that is, the average crystal grain size X 1 is 50 μm or less, the standard deviation S 1 satisfies [S 1 / X 1 <0.75], and the sharpness K 1 is a laminate of non-oriented electrical steel sheets with a sharpness of 20.0 or less. The stator core is the above-mentioned second non-oriented electrical steel sheet, that is, the average crystal grain size X 2 is 80 μm or more, the standard deviation S 2 satisfies [S 2 / X 2 <0.75], and the sharpness K 2 is a laminate of non-oriented electrical steel sheets with a sharpness of 3.00 or less. Since the rotor core of this motor core has high fatigue strength and the stator core has excellent magnetic characteristics, miniaturization and high output can be easily achieved.

[0128] <Manufacturing method of non-oriented electrical steel sheet>

[0129] Next, the manufacturing method of the non-oriented electrical steel sheet of the present invention will be described.

[0130] Generally speaking, a method is used in which a steel billet having the above-described composition is used as the starting billet, and a hot rolling process, an optional hot rolled sheet annealing process, a pickling process, a cold rolling process, and an annealing process are carried out in sequence. Thus, the first non-oriented electrical steel sheet of the present invention described above can be obtained. In addition, by performing heat treatment on the first non-oriented electrical steel sheet, the second non-oriented electrical steel sheet of the present invention can be obtained. In the present invention, if the composition of the steel billet, the conditions of the cold rolling process and the annealing process, and the conditions of the heat treatment process are within a specified range, the other conditions are not particularly limited. It should be noted that there is no particular limitation on the method for manufacturing a motor core, and a generally well-known method can be used.

[0131] (Steel billet)

[0132] The steel billet is not particularly limited as long as it is a steel billet having the composition described for the non-oriented electrical steel sheet.

[0133] As the melting method of the steel billet, there is no particular limitation, and a well-known melting method such as using a converter or an electric furnace can be adopted. Considering problems such as productivity, it is preferably made into a slab (steel billet) by continuous casting after melting, but it can also be made into a slab by a known casting method such as ingot - blooming rolling or thin slab continuous casting.

[0134] (Hot rolling process)

[0135] The hot rolling process is a process of obtaining a hot rolled sheet by performing hot rolling on a steel billet having the above-described composition. The hot rolling process is not particularly limited as long as it is a process of heating a steel billet having the above-described composition and performing hot rolling to obtain a hot rolled sheet of a specified size, and a common hot rolling process can be applied.

[0136] As a common hot rolling process, for example, the following hot rolling process can be cited: heating the steel billet to a temperature of 1000°C to 1200°C, performing hot rolling on the heated steel billet at a finish rolling exit side temperature of 800°C to 950°C, after the hot rolling is completed, performing appropriate post-rolling cooling (for example, cooling at an average cooling rate of 20°C / s to 100°C / s in a temperature range of 450°C to 950°C), and winding at a winding temperature of 400°C to 700°C to form a hot rolled sheet of a specified size and shape.

[0137] (Hot rolled sheet annealing process)

[0138] The hot rolled sheet annealing process is a process of annealing the hot rolled sheet by heating and maintaining a high temperature. The hot rolled sheet annealing process is not particularly limited, and a common hot rolled sheet annealing process can be applied. It should be noted that this hot rolled sheet annealing process is not essential and can also be omitted.

[0139] (Pickling process)

[0140] The pickling process is a process of pickling the hot-rolled sheet after the above-mentioned hot-rolling process or any of the above-mentioned hot-rolled sheet annealing processes. The pickling process is not particularly limited as long as it can pickle the hot-rolled sheet to a degree that enables cold rolling of the pickled steel sheet. For example, a common pickling process using hydrochloric acid or sulfuric acid can be applied. In the case where the above-mentioned hot-rolled sheet annealing process is carried out, this pickling process can be continuously carried out in the same production line as the hot-rolled sheet annealing process, or can be carried out on other production lines.

[0141] (Cold rolling process)

[0142] The cold rolling process is a process of cold rolling the pickled hot-rolled sheet (pickled sheet). More specifically, in the cold rolling process, the pickled hot-rolled sheet is cold-rolled under the conditions that the work roll diameter D in the final pass is 150 mmφ or more, the reduction ratio r in the final pass is 15% or more, and the strain rate ε m is 100 s -1 ~1300 s -1 to obtain a cold-rolled sheet. It should be noted that in the cold rolling process, as long as the above-mentioned cold rolling conditions are satisfied, the cold-rolled sheet of a specified size can also be made by cold rolling two or more times with intermediate annealing as needed. As the conditions for the intermediate annealing at this time, there is no particular limitation, and common intermediate annealing can be applied.

[0143] [Work roll diameter D in the final pass: 150 mmφ or more]

[0144] In the cold rolling process, the work roll diameter D in the final pass is made 150 mmφ or more. The reason for making the work roll diameter D in the final pass 150 mmφ or more is as follows: to make the sharpness K of the crystal grain size distribution in the obtained first non-oriented electromagnetic steel sheet 1 be 20.0 or less and form a desired steel sheet structure.

[0145] When the work roll diameter D in the final pass is less than 150 mmφ, since it is far from the state of plane compression, the non-uniformity of the shear strain per crystal grain is enhanced compared with the case of a large work roll diameter. Due to this non-uniformity of the shear strain, a certain amount is generated in the regions where the nucleation frequency of the recrystallization nuclei in the subsequent annealing process is very high and very low. Therefore, the sharpness of the crystal grain size distribution of the annealed sheet becomes large.

[0146] On the other hand, when the work roll diameter D in the final pass is 150 mmφ or more, the sharpness K of the crystal grain size distribution after the annealing process described later 1 becomes 20.0 or less. As a result, a desired steel sheet structure can be obtained.

[0147] The working roll diameter D in the final pass is preferably 170 mm φ or more, more preferably 200 mm φ or more. It should be noted that the upper limit of the working roll diameter D in the final pass is not particularly limited. When the roll diameter is too large, the rolling load increases, so it is preferably 700 mm φ or less.

[0148] [Reduction ratio r in the final pass: 15% or more]

[0149] In the cold rolling process, the reduction ratio r in the final pass is made 15% or more. The reason for making the reduction ratio r in the final pass 15% or more is as follows: The effects of a series of cold rolling controls can be achieved, and a desired steel plate structure can be formed.

[0150] When the reduction ratio r in the final pass is less than 15%, it is difficult to control the structure after annealing because the reduction ratio is too low. On the other hand, when the reduction ratio r in the final pass is 15% or more, a series of cold rolling control effects are exerted. As a result, a desired steel plate structure can be obtained.

[0151] The reduction ratio r in the final pass is preferably 20% or more. It should be noted that the upper limit of the reduction ratio r in the final pass is not particularly limited, but an excessively high reduction ratio requires a large equipment capacity, and the shape control of the cold rolled sheet also becomes difficult. Therefore, the reduction ratio r in the final pass is usually 50% or less.

[0152] [Strain rate ε in the final pass m : 100 s -1 ~1300 s -1

[0153] In the cold rolling process, the strain rate ε in the final pass m is made 100 s -1 ~1300 s -1 . The reason for making the strain rate ε in the final pass m be 100 s -1 ~1300 s -1 is as follows: Fracture during rolling is suppressed, and the sharpness K 1 of the crystal grain size distribution in the obtained non-oriented electrical steel sheet is made 20.0 or less, and a desired steel plate structure is formed.

[0154] The strain rate ε in the final pass m is less than 100 s -1 When, the non-uniformity of the shear strain per crystal grain of the cold rolled sheet is enhanced, emphasizing the site dependence of nucleation and grain growth in the subsequent annealing process. Therefore, the sharpness K 1 of the crystal grain size distribution of the annealed sheet becomes large. The reason for this is not clear, but the inventors et al. speculate that it is because the strain rate ε m ​is low, the flow stress is reduced, and strain is likely to concentrate in grains with crystal orientations that are easily deformed, making the strain distribution non-uniform. On the other hand, when the strain rate ε m exceeds 1300 s -1 , the flow stress increases excessively, and brittle fracture during rolling is likely to occur.

[0155] The strain rate ε of the final pass m is 100 s -1 ~1300 s -1 When it is, fracture during rolling is suppressed, and the sharpness K of the crystal grain size distribution after the annealing process described below 1 becomes 20.0 or less. As a result, a desired steel plate structure can be obtained.

[0156] The strain rate ε of the final pass m is preferably 150 s -1 or more, and preferably 1000 s -1 or less.

[0157] It should be noted that the strain rate ε in each pass during cold rolling m is derived using the following Ekelund's approximate formula.

[0158] [Mathematical formula 1]

[0159]

[0160] where v R is the roll circumferential speed (mm / s), R’ is the roll radius (mm), h 1 is the roll inlet side plate thickness (mm), and r is the reduction ratio (%).

[0161] (Annealing process)

[0162] The annealing process is a process of annealing the cold-rolled sheet that has undergone the cold rolling process. More specifically, in the annealing process, the cold-rolled sheet that has undergone the cold rolling process is heated to an annealing temperature T of 700°C to 850°C under the condition that the average heating rate V from 500°C to 700°C 1 is 10°C / s or more 2 , and then cooled to obtain a cold-rolled annealed sheet (the first non-oriented electrical steel sheet). It should be noted that after the annealing process, insulation coating can be applied to the surface. As the coating method and coating type, there are no particular limitations, and common insulation coating processes can be applied.

[0163] [Average heating rate V from 500°C to 700°C 1 : 10°C / s or more]

[0164] In the annealing process, the average heating rate V from 500°C to 700°C 1 is 10°C / s or more. The reason for making the average heating rate V 1 be 10°C / s or more is as follows: The standard deviation S of the crystal grain size distribution in the obtained non-oriented electrical steel sheet 1 satisfies the above formula (1), and a desired steel sheet structure is formed.

[0165] When the average heating rate V 1 is less than 10°C / s, due to excessive recovery, the generation frequency of recrystallization nuclei decreases, and the position dependence of the number of recrystallization nuclei becomes larger. As a result, fine grains and coarse grains coexist, and the standard deviation S of the crystal grain size distribution 1 becomes larger and does not satisfy the above formula (1).

[0166] On the other hand, when the average heating rate V 1 is 10°C / s or more, the generation frequency of recrystallization nuclei becomes higher, and the position dependence of the number of recrystallization nuclei becomes smaller. As a result, the standard deviation S of the crystal grain size distribution 1 becomes smaller and satisfies the above formula (1).

[0167] The average heating rate V from 500°C to 700°C 1 is preferably 20°C / s or more, more preferably 50°C / s or more. It should be noted that the upper limit of the average heating rate V 1 is not particularly limited, but if the heating rate is too high, temperature unevenness is likely to occur. Therefore, the average heating rate V 1 is preferably 500°C / s or less.

[0168] [Annealing temperature T 2 : 700°C to 850°C]

[0169] In the annealing process, the annealing temperature T 2 is 700°C to 850°C. The reason for making the annealing temperature T 2 be 700°C to 850°C is as follows.

[0170] When the annealing temperature T 2 is less than 700°C, grain growth is inhibited, so the position dependence of the number of recrystallization nuclei is emphasized, and a structure that retains the initial non-uniformity is formed. Therefore, the standard deviation S of the crystal grain size distribution 1 becomes larger. On the other hand, when the annealing temperature T 2 is 700°C or more, sufficient grain growth can occur, and the standard deviation S of the crystal grain size distribution 1 can satisfy the above formula (1), and a desired steel sheet structure can be obtained. The annealing temperature T 2 is preferably 750°C or more.

[0171] On the other hand, when the annealing temperature T 2 exceeds 850 °C, the recrystallized grains grow excessively, and the average crystal grain diameter X 1 cannot be made 50 μm or less. Therefore, the annealing temperature T 2 is made 850 °C or less. The annealing temperature T 2 is preferably 825 °C or less.

[0172] In the annealing process, it is heated to the above annealing temperature T 2 and then cooled. From the viewpoint of preventing uneven cooling, this cooling is preferably performed at a cooling rate of 50 °C / s or less.

[0173] (Heat treatment process)

[0174] The heat treatment process is a process of performing heat treatment on the cold-rolled annealed sheet (first non-oriented electromagnetic steel sheet) that has undergone the above annealing process. More specifically, in the heat treatment process, the cold-rolled annealed sheet (first non-oriented electromagnetic steel sheet) that has undergone the above annealing process is heated to a heat treatment temperature T 3 of 750 °C to 900 °C. By cooling after heating, a heat-treated sheet (second non-oriented electromagnetic steel sheet) can be obtained. It should be noted that the heat treatment process is usually performed on the stator core formed by laminating the above non-oriented electromagnetic steel sheets, but the same effect can also be obtained when performing the heat treatment on the above non-oriented electromagnetic steel sheets before lamination.

[0175] [Heat treatment temperature T 3 : 750 °C to 900 °C]

[0176] In the heat treatment process, the heat treatment temperature T 3 is made 750 °C to 900 °C. The reason for making the heat treatment temperature T 3 750 °C to 900 °C is as follows.

[0177] When the heat treatment temperature T 3 is less than 750 °C, the grain growth is insufficient, and the average crystal grain diameter X 2 in the obtained second non-oriented electromagnetic steel sheet cannot be made 80 μm or more. Therefore, the heat treatment temperature T 3 is made 750 °C or more. The heat treatment temperature T 3 is preferably 775 °C or more.

[0178] On the other hand, when the heat treatment temperature exceeds 900 °C, the selectivity of grain growth is emphasized, and the skewness of the crystal grain diameter distribution becomes too large. As a result, the sharpness K 2 of the crystal grain diameter distribution in the obtained second non-oriented electromagnetic steel sheet is not 3.00 or less. Therefore, the heat treatment temperature T 3 is made 900 °C or less. The heat treatment temperature T3 Preferably, it is below 875 °C.

[0179] By performing the above heat treatment process, the microstructure of the above-described second non-oriented electrical steel sheet is obtained, that is, the average crystal grain diameter X 2 is 80 μm or more, and the standard deviation S 2 satisfies [S 2 / X 2 <0.75], and the kurtosis K 2 is 3.00 or less for the microstructure of the steel sheet. This change in the microstructure is affected by the microstructure of the steel sheet before the heat treatment process. That is, in order to perform the heat treatment process to obtain a standard deviation S 2 satisfies [S 2 / X 2 <0.75], and the kurtosis K 2 is 3.00 or less for the microstructure, the steel sheet before the heat treatment process requires a standard deviation S 1 satisfies [S 1 / X 1 <0.75], and the kurtosis K 1 is 20.0 or less.

[0180] Examples

[0181] Examples are listed below to specifically illustrate the present invention. However, the present invention is not limited thereto.

[0182] <Manufacture of cold-rolled annealed sheet (first non-oriented electrical steel sheet)>

[0183] The molten steel having the composition shown in Table 1 was melted by a generally known method and continuously cast to form a slab (steel billet) with a thickness of 230 mm.

[0184] The obtained slab was hot-rolled to obtain a hot-rolled sheet with a thickness of 2.0 mm. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing and pickling by a known method, and then cold-rolled to the thickness shown in Table 2 to obtain a cold-rolled sheet.

[0185] The obtained cold-rolled sheet was annealed under the conditions shown in Table 2, and then coated by a known method to obtain a cold-rolled annealed sheet (first non-oriented electrical steel sheet).

[0186] <Manufacture of heat-treated sheet (second non-oriented electrical steel sheet)>

[0187] The obtained cold-rolled annealed sheet was heat-treated under the conditions shown in Table 2 to obtain a heat-treated sheet (second non-oriented electrical steel sheet).

[0188] <Manufacture of motor core>

[0189] A motor core is obtained by combining a rotor core formed by laminating cold-rolled annealed sheets (first non-oriented electromagnetic steel sheets) using a known method and a stator core formed by laminating heat-treated sheets (second non-oriented electromagnetic steel sheets).

[0190] <Evaluation>

[0191] (Observation of microstructure)

[0192] Specimens for microstructure observation are taken from the obtained cold-rolled annealed sheets and heat-treated sheets. Then, the taken specimens are thinned and mirror-finished by chemical polishing so that the position equivalent to 1 / 4 of the plate thickness on the rolling surface (ND surface) becomes the observation surface. Electron backscatter diffraction (EBSD) measurement is performed on the mirror-finished observation surface to obtain local orientation data. At this time, for the cold-rolled annealed sheets, the step size is set to 2 μm and the measurement area is set to 4 mm 2 Above, for the heat-treated sheets, the step size is set to 10 μm and the measurement area is set to 100 mm 2 Above. The width of the measurement area is appropriately adjusted so that the number of grains becomes 5000 or more in subsequent analysis. It should be noted that the measurement can be performed on the entire area in one scan, or the results of multiple scans can be combined using the Combo Scan function. Using analysis software: OIM Analysis 8, the obtained local orientation data is analyzed.

[0193] Before data analysis, the average grain data points are screened using the Partition Properties of the analysis software under the condition of Formula: GCI[&;5.000,2,0.000,0,0,8.0,1,1,1.0,0;]>0.1 to exclude data points that are not suitable for analysis. At this time, the effective data points are 97% or more.

[0194] For the data adjusted as above, as the definition of grain boundaries, the Grain Tolerance Angle is set to 5°, the Minimum Grain Size is set to 2, the Minimum AntiGrain Size is set to 2, and both the Multiple RowsRequirement and Anti-Grain Multiple Rows Requirement are set to OFF, and the following analysis is performed.

[0195] For the data that has undergone preprocessing, the grain information is output using the Export Grain File function. The Grain Size (Diameterin microns) of Grain File Type 2 is used as the crystal grain diameter (x i)。For the information of all the obtained grains, the average crystal grain size, standard deviation, and kurtosis are calculated respectively using the following formula. For the obtained average crystal grain size, standard deviation, and kurtosis, they are X 1 , S 1 and K 1 for the cold-rolled annealed sheet, and X 2 , S 2 and K 2 for the heat-treated sheet.

[0196] [Mathematical formula 2]

[0197] Average crystal grain size

[0198] Standard deviation

[0199] Kurtosis

[0200] In the above formula, n is the number of grains, and x i is the crystal grain size data for each (i: 1, 2, ···, n).

[0201] (Evaluation of fatigue characteristics)

[0202] Take a tensile fatigue test piece with the rolling direction as the long side direction from the obtained cold-rolled annealed sheet (No. 1 test piece according to JIS Z2275: 1978, with the same shape as b: 15 mm, R: 100 mm) for the fatigue test. Here, the end face of the test piece is machined to be smooth by mechanical processing. The above fatigue test is carried out under the conditions of test temperature: room temperature (25 °C), tension-tension (pendulum), stress ratio (= minimum stress / maximum stress): 0.1, and frequency: 20 Hz. The maximum stress at which no fatigue fracture occurs after 10 7 cycles is measured as the fatigue limit. It should be noted that the test results are evaluated as excellent fatigue characteristics when the fatigue limit is 500 MPa or more.

[0203] (Evaluation of magnetic characteristics)

[0204] Take a magnetic measurement test piece with a width of 30 mm and a length of 280 mm in the rolling direction and the direction perpendicular to the rolling direction from the obtained heat-treated sheet. According to JIS C2550-1: 2011, the iron loss W 10 / 400 of the heat-treated sheet is measured by the Epstein method. When W 10 / 400 ≤ 13.0 (W / kg), it is evaluated that the iron loss characteristic is good.

[0205] The above results are shown in Table 3.

[0206]

[0207]

[0208] [Table 2]

[0209]

[0210] Note) The underlined part indicates outside the scope of the invention

[0211] Table 2 (continued)

[0212]

[0213] Note) The underlined part indicates outside the scope of the invention

[0214] [Table 3]

[0215]

[0216] Note) The underlined part indicates outside the scope of the invention

[0217] Table 3 (continued)

[0218]

[0219] Note) The underlined part indicates outside the scope of the invention

[0220] From the results of Table 3, it can be seen that the non-oriented electromagnetic steel sheets based on the present invention can all exhibit excellent fatigue strength and excellent iron loss characteristics. It should be noted that the motor core obtained by combining the rotor core formed by laminating the cold-rolled annealed sheets based on the present invention and the stator core formed by laminating the heat-treated sheets has excellent fatigue characteristics.

Claims

1. An non-oriented electromagnetic steel sheet, characterized in that, it has the following composition components: containing by mass % C: 0.01% or less, Si: 2.0% or more and less than 4.5%, Mn: 0.05% - 5.00%, P: 0.1% or less, S: 0.01% or less, Al: 3.0% or less and N: 0.0050% or less, and, Si + Al is less than 4.5%, and the balance is Fe and inevitable impurities, And, regarding the crystal grains in the steel plate, the average crystal grain diameter X 1 is 50 μm or less, and the standard deviation S of the crystal grain diameter distribution 1 satisfies the following formula (1), and the kurtosis K of the crystal grain diameter distribution 1 is 20.0 or less. S 1 / X 1 <0.75…(1).

2. The non-oriented electromagnetic steel sheet according to claim 1, wherein, the composition further contains Co: 0.0005% - 0.0050% by mass %.

3. The non-oriented electromagnetic steel sheet according to claim 1 or 2, wherein, the composition further contains Cr: 0.05% - 5.00% by mass %.

4. The non-oriented electromagnetic steel sheet according to any one of claims 1 to 3, wherein, the composition further contains Ca: 0.001% - 0.100%, Mg: 0.001% - 0.100% and REM: 0.001% - 0.100% in any one or more than two kinds.

5. The non-oriented electromagnetic steel sheet according to any one of claims 1 to 4, wherein, the composition further contains Sn: 0.001% - 0.200% and Sb: 0.001% - 0.200% in any one or two kinds.

6. The non-oriented electromagnetic steel sheet according to any one of claims 1 to 5, wherein, the composition further contains Cu: 0% - 0.5%, Ni: 0% - 0.5%, Ti: 0% - 0.005%, Nb: 0% - 0.005%, V：0％～0.010％、 Ta: 0% - 0.002%, B：0％～0.002％、 Ga: 0% - 0.005%, Pb: 0% - 0.002%, Zn: 0% - 0.005%, Mo: 0% - 0.05%, W：0％～0.05％、 Ge: 0% - 0.05% and As: 0% - 0.05% in any one or more than two kinds.

7. A non-oriented electromagnetic steel sheet, characterized in that, it has the composition according to any one of claims 1 to 6, Moreover, regarding the crystal grains in the steel plate, the average crystal grain diameter X 2 is 80 μm or more, and the standard deviation S of the crystal grain diameter distribution 2 satisfies the following formula (2), and the kurtosis K of the crystal grain diameter distribution 2 is 3.00 or less. S 2 / X 2 <0.75…(2).

8. A method for manufacturing a non-oriented electromagnetic steel sheet, which is a method for manufacturing the non-oriented electromagnetic steel sheet according to any one of claims 1 to 6, and has the following steps: A hot rolling step of performing hot rolling on a steel billet having the composition according to any one of claims 1 to 6 to obtain a hot rolled sheet; A pickling step of pickling the hot rolled sheet; Cold rolling process, for the hot-rolled sheet that has undergone the pickling, with the working roll diameter D of the final pass being 150 mmφ or more, the reduction ratio r of the final pass being 15% or more, and the strain rate ε m being 100 s -1 to 1300 s -1 conditions, cold rolling is carried out to obtain a cold-rolled sheet; and Annealing process, heating the cold-rolled sheet at an average heating rate V of 500 °C to 700 °C 1 to an annealing temperature T of 700 °C to 850 °C under the condition that V is 10 °C / s or more 2 and then cooling to obtain a cold-rolled annealed sheet belonging to a non-oriented electrical steel sheet.

9. A method for manufacturing a non-oriented electromagnetic steel sheet, which is a method for manufacturing the non-oriented electromagnetic steel sheet according to claim 7, and includes a heat treatment step: heating the non-oriented electromagnetic steel sheet according to any one of claims 1 to 6 at a heat treatment temperature T of 750°C to 900°C 3 for heating.

10. A motor core is composed of a rotor core and a stator core. The rotor core is a laminate of the non-oriented electromagnetic steel sheet according to any one of claims 1 to 6, and the stator core is a laminate of the non-oriented electromagnetic steel sheet according to claim 7.

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