Non-oriented electromagnetic steel sheet and method for manufacturing the same

Abstract

When a slab containing prescribed amounts of C, Si, Mn, Al, N, and Cr is subjected to hot rolling, annealing of the hot-rolled sheet, cold rolling, and final annealing in a continuous annealing furnace to produce a non-oriented electromagnetic steel sheet, by setting the maximum temperature reached in the final annealing to less than 900°C, setting the average cooling rate from the temperature of (the maximum temperature reached - 50°C) to 500°C during the cooling of the final annealing to 40°C / s or more, and setting a parameter ε / t defined by the plastic elongation ε (%) in the rolling direction before and after the final annealing and the soaking time t (s) of the final annealing to 0.10 or more, a non-oriented electromagnetic steel sheet having a high strength with a yield stress of 500 MPa or more, a low loss in the high frequency region, and an average grain size of ferrite of less than 50 μm, a compressive residual stress σ S , σ C in the sheet width direction of the steel sheet surface and the center portion of the sheet thickness of 2.0 MPa or more, respectively, is obtained using an X-ray stress measurement method.

Description

Technical Field

[0001] This invention relates to a high-strength, non-oriented electromagnetic steel sheet with low eddy current loss in the high-frequency region and a method for manufacturing the same. Background Technology

[0002] Drive motors for electric vehicles (EVs), hybrid electric vehicles (HEVs), and compressor motors for high-efficiency air conditioners not only require high efficiency but also miniaturization from the perspectives of space saving and weight reduction. Furthermore, high-speed rotation is necessary to ensure output. The core material of these motors primarily uses non-oriented electromagnetic steel plates, which are soft magnetic materials.

[0003] Since the centrifugal force generated by the rotor is proportional to the square of the rotational speed, suppressing rotor damage has become a key challenge in the design of high-speed rotating motors. Therefore, in high-speed rotating motors, the steel sheet used as the rotor material is strengthened by refining the grain size or retaining non-recrystallized structures.

[0004] Furthermore, the rotor's excitation waveform contains high-order harmonics (slot harmonics) of approximately 1Hz to 10kHz caused by the motor's construction, thus increasing iron losses due to these harmonics becomes a problem. To address this issue, for non-oriented electromagnetic steel sheets used as core materials, in addition to requiring high strength, there is a strong demand for reducing iron losses in the high-frequency region.

[0005] The iron loss W of non-oriented electromagnetic steel sheet is the hysteresis loss W. h With eddy current loss W e The sum of these values, and proportional to the first and second powers of the frequency respectively, indicates that the eddy current loss W in the high-frequency region... e It has become dominant. As a countermeasure, research has been conducted on reducing eddy current losses by utilizing high alloying (for example, see Patent Documents 1 and 2).

[0006] However, while high alloying is an effective method for reducing iron loss in the high-frequency region of non-oriented electromagnetic steel sheets, it also increases the strength of the steel, leading to other problems such as making cold rolling more difficult. Therefore, Patent Document 1 proposes using warm rolling in cold rolling, and Patent Document 2 proposes using Mn to suppress the increase in steel strength.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2014-210978

[0010] Patent Document 2: Japanese Patent Application Publication No. 2008-231504 Summary of the Invention

[0011] However, the warm rolling method proposed in Patent Document 1 has the problem that excessively high steel plate temperature at the start of rolling leads to deterioration of the rolled steel plate shape, which limits its application in industrial production. Furthermore, the method using Mn proposed in Patent Document 2 requires the addition of a large amount of Mn to achieve sufficient iron loss reduction, resulting in increased raw material costs and instability in iron loss due to the formation of Mn carbides.

[0012] The present invention was made in view of the above-mentioned problems existing in the prior art. Its object is to reduce eddy current losses in the high-frequency region by means other than high alloying, to provide a high-strength non-oriented electromagnetic steel sheet with low iron loss in the high-frequency region, and to provide an advantageous manufacturing method thereof.

[0013] To address the aforementioned issues, the inventors, based on the premise of using a technique that retains non-recrystallized structures as a method to achieve high strength, and focusing on the technique used in oriented electromagnetic steel sheets to reduce iron loss by applying tensile stress to the steel sheet, repeatedly and thoroughly studied solutions for reducing eddy current losses in non-oriented electromagnetic steel sheets. Their results showed that by controlling the residual stress value of the product steel sheet within an appropriate range, even at high strength, eddy current losses in the high-frequency region can be reduced, thus developing this invention.

[0014] The present invention, based on the above insights, is a non-oriented electromagnetic steel sheet, characterized in that the average grain size of ferrite is less than 50 μm, the yield strength is 500 MPa or more, and the compressive residual stress σ in the width direction of the steel sheet surface and the center of the sheet thickness, as measured by X-ray stress measurement, is [not specified in the original text]. S σ C These are all above 2.0 MPa. Here, the aforementioned X-ray stress measurement method utilizes the 2θ-sinθ of the α-Fe(211) diffraction peak. 2 ψ method.

[0015] The non-oriented electromagnetic steel sheet of the present invention is characterized by having the following composition: containing C: 0-0.0050 wt%, Si: 2.0-5.0 wt%, Mn: 0-3.0 wt%, P: 0-0.2 wt%, S: 0-0.0050 wt%, Al: 0-3.0 wt%, N: 0-0.0050 wt%, Cr: 0-3.0 wt%, and O: 0-0.0050 wt%, with the remainder consisting of Fe and unavoidable impurities.

[0016] Furthermore, the non-oriented electromagnetic steel sheet of the present invention is characterized in that, in addition to the above-mentioned composition, it further contains at least one component from groups A to D:

[0017] Group A: At least one of Sn: 0–0.20% by mass and Sb: 0–0.20% by mass;

[0018] Group B: At least one of Ca: 0–0.01% by mass, Mg: 0–0.01% by mass, and REM: 0–0.05% by mass;

[0019] Group C: At least one of Cu: 0–0.5% by mass and Ni: 0–0.5% by mass;

[0020] Group D: At least one of Ge: 0–0.05% by mass, As: 0–0.05% by mass, and Co: 0–0.05% by mass.

[0021] Furthermore, the non-oriented electromagnetic steel sheet of the present invention is characterized in that, in addition to the above-mentioned composition, it further contains at least one component from the following groups E to I:

[0022] Group E: At least one of Ti: 0–0.005% by mass, Nb: 0–0.005% by mass, V: 0–0.010% by mass, and Ta: 0–0.002% by mass;

[0023] Group F: At least one of B: 0–0.002% by mass and Ga: 0–0.005% by mass;

[0024] Group G: Pb: 0–0.002% by mass;

[0025] Group H: Zn: 0–0.005% by mass;

[0026] Group I: At least one of Mo: 0 to 0.05% by mass and W: 0 to 0.05% by mass.

[0027] Furthermore, the non-oriented electromagnetic steel sheet of the present invention is characterized in that the non-recrystallized structure in the thickness section parallel to the rolling direction is 1% or more in terms of area.

[0028] Furthermore, the present invention provides a method for manufacturing a non-oriented electromagnetic steel sheet, characterized in that it is a method for manufacturing a non-oriented electromagnetic steel sheet according to any one of claims 1 to 5, wherein a slab having the composition described in any one of the above claims is hot-rolled, hot-rolled and annealed, and cold-rolled, and then finally annealed in a continuous annealing furnace, wherein the maximum temperature reached in the final annealing is set to be less than 900°C, and the average cooling rate from the temperature (maximum temperature reached - 50°C) to 500°C during the cooling process of the final annealing is set to be 40°C / s or more, and the parameter ε / t, defined by the plastic elongation ε (%) in the rolling direction before and after the final annealing and the soaking time t (s) of the final annealing, is set to 0.10 or more.

[0029] According to the present invention, high strength and low iron loss in the high-frequency range of non-oriented electromagnetic steel sheets can be achieved without using methods that adversely affect manufacturing costs and productivity, such as high alloying or thinning. Therefore, by using the non-oriented electromagnetic steel sheets of the present invention in the rotor cores of drive motors for EVs, HEVs, and compressor motors for high-efficiency air conditioners, it is highly beneficial to the high efficiency and miniaturization of motors. Attached Figure Description

[0030] Figure 1 It represents the ratio of plastic elongation ε (%) to soaking time t (s) during final annealing, ε / t, and the eddy current loss W in the rolling direction and width direction of the steel plate. e3 / 5k A coordinate graph showing the relationship between them.

[0031] Figure 2 It represents the ratio of plastic elongation ε (%) to soaking time t (s) during final annealing, ε / t, to the compressive residual stress σ in the width direction of the steel plate surface and the center of the plate thickness. S σ C A coordinate graph showing the relationship between them. Detailed Implementation

[0032] The experiment that served as the impetus for developing this invention will be described.

[0033] In the field of oriented electromagnetic steel sheets, it is known that applying tensile stress in the rolling direction of the steel sheet can refine magnetic domains and reduce eddy current losses. Therefore, the inventors have also investigated applying residual stress to the product steel sheet as a method to reduce eddy current losses in non-oriented electromagnetic steel sheets. Furthermore, using tension annealing in the final annealing process as a method to introduce residual stress into the aforementioned product steel sheet, the influence of residual stress on iron loss characteristics was investigated through the following experiments.

[0034] Steel with a composition of C: 0.0015 wt%, Si: 3.37 wt%, Mn: 0.40 wt%, P: 0.01 wt%, S: 0.0009 wt%, Al: 0.91 wt%, N: 0.0018 wt%, Cr: 0.02 wt%, and O: 0.0012 wt%, with the remainder consisting of Fe and unavoidable impurities, is melted in a vacuum furnace and cast into ingots. Then, it is hot-rolled to produce a hot-rolled sheet with a thickness of 1.5 mm. Next, the hot-rolled sheet is annealed at 1000°C for 30 s in a N2 atmosphere, followed by cold rolling to produce a cold-rolled sheet with a thickness of 0.3 mm. Then, a final annealing is performed in a mixed gas atmosphere with a vol% ratio of H2:N2 = 3:7, while applying a tensile stress of 5–20 MPa in the rolling direction of the cold-rolled sheet. In the aforementioned final annealing, the maximum reaching temperature is set to 780°C, and a homogenization treatment is performed within the temperature range of the maximum reaching temperature to (maximum reaching temperature - 10°C) for 1 to 30 seconds. Then, final annealing is carried out by gas cooling from (maximum reaching temperature - 50°C) to 500°C at an average cooling rate of 50°C / s to produce a product sheet. At this time, marking lines are drawn along the width direction on the surface of the steel sheet before final annealing, and the interval between the marking lines before and after final annealing is measured. The plastic elongation ε (%) in the rolling direction is calculated based on this difference. Here, the plastic elongation ε is the elongation at nominal strain. It should be noted that in this invention, the time spent within the temperature range of the maximum reaching temperature to (maximum reaching temperature - 10°C) is defined as the homogenization time.

[0035] From the product plate obtained as described above, Epstein test pieces with a width of 30 mm and a length of 100 mm were cut along the rolling direction and the plate width direction, respectively. The iron loss W was measured in the range of 50 to 5 kHz with a maximum magnetic flux density of 0.3 T and a frequency f of 50 to 5 kHz by the single-piece magnetic measurement method.

[0036] Next, based on the measured relationship between the frequency f and the iron loss value W, the hysteresis loss W is calculated using the two-frequency method described below. h and eddy current loss W e The separation is as follows: First, the relationship between W / f and f in the range of 50–1000 Hz is plotted, and the approximate formula W / f = af + b is obtained using the least squares method. Here, the coefficients a and the intercept b are constants. Since the intercept b is the hysteresis loss for each cycle, the hysteresis loss W at the frequency f can be obtained by multiplying the intercept b by the frequency f. h (f). On the other hand, eddy current loss W e (f) is the measured iron loss W at frequency f minus the aforementioned hysteresis loss W. h (f) yields W. e (f) = W - Wh (f) It should be noted that in this experiment, the eddy current loss W at a maximum magnetic flux density of 0.3T and a frequency of 5kHz is considered as an indicator of the increase in iron loss caused by higher harmonics. e3 / 5k To analyze the results.

[0037] In addition, to investigate the strength characteristics of the product sheet, JIS No. 5 specimens were taken from the final annealed steel sheet with the rolling direction as the tensile direction, and tensile tests were conducted according to JIS Z 2241 to determine the yield stress. The results showed small deviations in yield stress, with all specimens falling within the range of 520–540 MPa. Furthermore, the microstructure was revealed by etching a section of the sheet thickness perpendicular to the width direction (a section parallel to the rolling direction) using a solution of nitric acid and ethanol, and the average grain size (the average segment length of each crystal in the test section) was determined using a cutting method. The results showed small deviations in average grain size, all falling within the range of 17–20 μm.

[0038] Figure 1 The parameter ε / t, defined by the plastic elongation ε (%) during final annealing and the soaking time t (s), is related to the eddy current loss W in the rolling direction and width direction of the steel plate. e3 / 5k The relationship. According to this figure, by making ε / t greater than 0.10, although the W in the rolling direction... e3 / 5k Slightly increased, but W in the width direction of the plate e3 / 5k The average iron loss is significantly reduced in both the rolling direction and the width direction.

[0039] Next, the inventors investigated the reasons why eddy current losses were reduced by setting ε / t to 0.10 or higher as described above. The results showed a strong correlation between this reduction in eddy current losses and the residual stress of the product board. Here, the residual stress was measured using an X-ray stress measurement method, specifically using a Rigaku MSF-2M X-ray measuring device. The X-ray source was a Cr tube (kβ filter: V), with an output of 30 kV × 4 mV, using 2θ - sin 2 The ψ method (fixed ψ method with the same tilt) was used to perform X-ray scanning on a 7 mm × 7 mm area of ​​the specimen surface, and the intensity distribution corresponding to α-Fe(211) around 2θ = 156.4° was measured. The ψ angle was set to 12, 16, 20, 24, 28, 32, 36, 40, 44, and 48°, and the ψ oscillation angle was set to a range of ±3°. Next, the diffraction angle 2θ that shows the peak in the measured intensity distribution was taken as the diffraction angle 2θ at each ψ angle and plotted on 2θ - sin 2 On the ψ-line graph, the slope of the line is determined using the least squares method, and the residual stress σ (MPa) is calculated using the following formula.

[0040] σ=317.91·Δ2θ / Δsin 2 ψ

[0041] It should be noted that in 2θ-sin 2 In the ψ method, by changing the scanning plane of the X-rays, stress in any direction within the sample surface can be measured. The residual stress σ is measured at two locations: the surface of the specimen and the center of the plate thickness. The residual stress on the surface is set as σ. S The residual stress at the center of the plate thickness is set as σ. C It should be noted that the thickness at the center of the plate was measured by chemical grinding from one surface of the specimen to its center. Here, when the residual stress σ is positive, compressive residual stress exists in the material; conversely, when it is negative, tensile residual stress exists in the material.

[0042] The compressive residual stress σ in the width direction of the steel plate surface and the center of the plate thickness, as measured above. S σ C Iron loss W in the width direction of the plate e3 / 5k The relationship is shown in Figure 2 According to this Figure 2 and the above Figure 1 It can be seen that if the residual compressive stress increases, then the W in the width direction of the plate will increase. e3 / 5k reduce.

[0043] The mechanism is not yet fully understood, but the inventors have considered the following.

[0044] When a steel sheet is subjected to tensile stress during final annealing, plastic deformation and recrystallization / grain growth occur simultaneously at high temperatures. Here, because the yield stress during plastic deformation varies depending on the crystal orientation, the plastic strain introduced into each grain is non-uniform in polycrystalline materials, varying from grain to grain. Therefore, it is considered that if a state could be created where tensile residual stress is generated on the easily magnetized axis <100> and compressive residual stress is generated on other, less magnetized axes such as <110>, <111>, and <112>, it would be possible to refine the magnetic domains and reduce eddy current losses. Such conditions are achieved in tension annealing. In tension annealing, the final annealing process, it is important to plastically deform the steel at high temperature for a short time, followed by rapid cooling to prevent the release of residual stress and strain due to recrystallization or recovery.

[0045] This invention was developed based on the aforementioned new insights.

[0046] Next, the composition of the non-oriented electromagnetic steel sheet of the present invention will be described.

[0047] C: 0 to 0.0050% by mass

[0048] C is a component that deteriorates iron loss by forming carbides in the product board through magnetic aging. Therefore, in order to suppress the above-mentioned magnetic aging, C is set in the range of 0 to 0.0050% by mass. Preferably, it is in the range of 0.0001 to 0.0020% by mass.

[0049] Si: 2.0–5.0% by mass

[0050] Si has the effect of increasing the resistivity of steel and reducing iron loss. In addition, it also has the effect of increasing the strength of steel through solid solution strengthening. From the viewpoint of achieving the above-mentioned low iron loss and high strength, the lower limit of Si is set to 2.0% by mass. On the other hand, if Si is greater than 5.0% by mass, rolling becomes difficult, therefore the upper limit is set to 5.0% by mass. A range of 3.5 to 5.0% by mass is preferred. In particular, from the viewpoint of ensuring an excellent balance between strength and iron loss, a range of 3.5 to 4.5% by mass is preferred.

[0051] Mn: 0–3.0% by mass

[0052] Mn has the effect of increasing the resistivity of steel and reducing iron loss. However, if the content is greater than 3.0% by mass, the precipitation of carbonitriding compounds will worsen the iron loss. Therefore, Mn is added in the range of 0 to 3.0% by mass. It should be noted that in order to reliably obtain the above-mentioned iron loss reduction effect, it is preferable to add 0.3% by mass or more, and from the viewpoint of suppressing the formation of carbonitriding compounds, the upper limit is preferably 2.0% by mass.

[0053] P: 0–0.2% by mass

[0054] P is a component used to adjust the strength of steel and can be added appropriately. However, if P is greater than 0.2% by mass, the steel becomes brittle and rolling becomes difficult. Therefore, the P content is set in the range of 0 to 0.2% by mass. It should be noted that when P is not used to adjust strength, it is preferable to be less than 0.02% by mass; on the other hand, when P is used, it is preferable to be in the range of 0.02 to 0.10% by mass.

[0055] S: 0 to 0.0050% by mass

[0056] Sulfide (S) is a harmful component that precipitates fine sulfides, hindering grain growth and increasing iron loss. In particular, if the content exceeds 0.0050% by mass, the aforementioned adverse effects become significant; therefore, the S content is set in the range of 0 to 0.0050% by mass. The preferred upper limit is 0.0020% by mass.

[0057] Al: 0–3.0% by mass

[0058] Al has the effect of increasing the resistivity of steel and reducing iron loss. In addition, it also has the effect of increasing the strength of steel through solid solution strengthening. However, if the content exceeds 3.0% by mass, rolling becomes difficult; therefore, the Al content is set in the range of 0 to 3.0% by mass. A range of 1.2 to 3.0% by mass is preferred. In particular, from the viewpoint of ensuring a good balance between strength and iron loss, a range of 1.2 to 2.5% by mass is more preferred. On the other hand, since Al is a component that easily produces voids during casting and solidification, it is preferable to limit it to 0.01% by mass or less when reusability is important.

[0059] N: 0 to 0.0050% by mass

[0060] Nitrogen (N) is a harmful component that precipitates fine nitrides, hindering grain growth and increasing iron loss. In particular, if the content exceeds 0.0050% by mass, the aforementioned adverse effects become significant; therefore, the N content is set in the range of 0 to 0.0050% by mass. A preferred upper limit is 0.0020% by mass.

[0061] Cr: 0–3.0% by mass

[0062] Cr has the effect of increasing the resistivity of steel and reducing iron loss. However, if the content is greater than 3.0% by mass, the precipitation of carbonitriding compounds will worsen the iron loss. Therefore, the Cr content is set in the range of 0 to 3.0% by mass. It should be noted that when the Cr content is less than 0.3% by mass, the above-mentioned iron loss reduction effect is small. Therefore, when iron loss is a concern, it is preferable to add 0.3% by mass or more. In addition, from the viewpoint of suppressing the formation of carbonitriding compounds, an upper limit of 2.0% by mass is preferred.

[0063] O: 0 to 0.0050% by mass

[0064] O is a harmful component that forms oxide inclusions, hinders grain growth, and increases iron loss. In particular, if the content exceeds 0.0050% by mass, the aforementioned adverse effects become significant. Therefore, the O content is set in the range of 0 to 0.0050% by mass. It should be noted that the preferred upper limit is 0.0020% by mass.

[0065] In addition to the above-mentioned components, the non-oriented electromagnetic steel sheet of the present invention may further contain the following components according to the desired characteristics.

[0066] At least one of Sn: 0–0.20% by mass and Sb: 0–0.20% by mass

[0067] Sn and Sb have the effect of improving recrystallization texture and reducing iron loss, and can be added appropriately. However, even if the addition is greater than 0.20% by mass, the above effects are saturated, so the upper limit for each is preferably 0.20% by mass. More preferably, each is in the range of 0.005 to 0.01% by mass.

[0068] At least one of Ca: 0–0.01% by mass, Mg: 0–0.01% by mass, and REM: 0–0.05% by mass.

[0069] Ca, Mg, and REM (rare earth metals) reduce fine sulfides by forming stable sulfides, thus improving grain growth and iron loss. However, excessive addition leads to increased iron loss. Therefore, when added, the upper limits are preferably Ca: 0.01 wt%, Mg: 0.010 wt%, and REM: 0.05 wt%. More preferably, the range is Ca: 0.001–0.005 wt%, Mg: 0.0005–0.003 wt%, and REM: 0.005–0.03 wt%.

[0070] In addition, the non-oriented electromagnetic steel sheet of the present invention may contain the following components in the following range, in addition to the above-mentioned components.

[0071] At least one of Cu: 0 to 0.5% by mass and Ni: 0 to 0.5% by mass

[0072] Cu and Ni are effective components for improving the toughness of steel and can be added appropriately. However, even if each is added at more than 0.5% by mass, the above effects will saturate, so the upper limit for each is preferably 0.5% by mass. More preferably, each is in the range of 0.01 to 0.1% by mass.

[0073] At least one of Ge: 0-0.05% by mass, As: 0-0.05% by mass, and Co: 0-0.05% by mass.

[0074] Ge, As, and Co are effective components for increasing magnetic flux density and reducing iron loss, and can be added appropriately. However, even if each is added at more than 0.05% by mass, the above effects will saturate, so the upper limit for each is preferably 0.05% by mass. More preferably, each is in the range of 0.002 to 0.01% by mass.

[0075] Furthermore, the non-oriented electromagnetic steel sheet of the present invention may contain, in addition to the above-mentioned components, the following components within the following range.

[0076] At least one of the following: Ti: 0–0.005 wt%, Nb: 0–0.005 wt%, V: 0–0.010 wt%, and Ta: 0–0.002 wt%.

[0077] Ti, Nb, V, and Ta are harmful components that form fine carbonitrides and increase iron loss. In particular, the adverse effects become significant if the values ​​exceed the aforementioned upper limits. Therefore, it is preferable that each of Ti, Nb, V, and Ta contains components in the range of 0–0.005% by mass, Nb: 0–0.005% by mass, V: 0–0.010% by mass, and Ta: 0–0.002% by mass. It should be noted that more preferred upper limits are 0.002% by mass for Ti, 0.002% by mass for Nb, 0.005% by mass for V, and 0.001% by mass for Ta.

[0078] At least one of B: 0 to 0.002% by mass and Ga: 0 to 0.005% by mass

[0079] B and Ga are harmful components that form fine nitrides and increase iron loss. In particular, the adverse effects become significant if the levels exceed the aforementioned upper limits. Therefore, it is preferable to add B and Ga in the range of 0 to 0.002% by mass for B and 0 to 0.005% by mass for Ga, respectively. It should be noted that more preferred upper limits are 0.001% by mass for B and 0.002% by mass for Ga.

[0080] Pb: 0–0.002% by mass

[0081] Pb is a harmful component that forms fine Pb particles and increases iron loss. In particular, if it exceeds 0.002% by mass, the aforementioned adverse effects become significant. Therefore, it is preferable to have Pb in the range of 0 to 0.002% by mass. It should be noted that a more preferred upper limit is 0.001% by mass.

[0082] Zn: 0–0.005% by mass

[0083] Zn is a harmful component that increases iron loss by adding fine inclusions, and in particular, the adverse effects become significant if it exceeds 0.005% by mass. Therefore, a Zn content in the range of 0 to 0.005% by mass is preferred. It should be noted that a more preferred upper limit is 0.003% by mass.

[0084] At least one of Mo: 0 to 0.05% by mass and W: 0 to 0.05% by mass

[0085] Mo and W are harmful components that form fine carbides and increase iron loss. In particular, if their content exceeds the above-mentioned upper limit, the adverse effects become significant. Therefore, it is preferable that Mo and W each contain a concentration in the range of 0 to 0.05% by mass and 0 to 0.05% by mass, respectively. It should be noted that a more preferred upper limit is 0.02% by mass for Mo and 0.02% by mass for W.

[0086] The non-oriented electromagnetic steel sheet of the present invention, apart from the above-mentioned components, is substantially composed of Fe and unavoidable impurities.

[0087] Next, the non-oriented electromagnetic steel sheet of the present invention will be described.

[0088] Yield stress: above 500 MPa

[0089] First, to prevent damage to the motor during high-speed rotation, the non-oriented electromagnetic steel sheet of this invention requires a yield stress of 500 MPa or higher. This yield stress is obtained by performing a tensile test on a JIS 5 tensile specimen with the rolling direction as the tensile direction, according to JIS Z 2241. It should be noted that if discontinuous yielding is confirmed, the upper yield point is taken as the yield stress; otherwise, 0.2% of the endurance is taken as the yield stress. Preferably, the yield stress is in the range of 600 to 800 MPa.

[0090] Average ferrite grain size: less than 50 μm

[0091] Furthermore, to achieve the aforementioned high strength, the average ferrite grain size in the non-oriented electromagnetic steel sheet of the present invention needs to be less than 50 μm. When the ferrite grain size is 50 μm or more, the resulting reduction in strength and damage to the rotor becomes a problem. Preferably, it is 25 μm or less. It should be noted that the above-mentioned average grain size is the value of the average grain size (average segment length of each crystal in the test segment) measured by a cutting method, which is the microstructure revealed by etching a section of the sheet thickness perpendicular to the sheet width direction (the section of the sheet thickness in the rolling direction) with a nitric acid ethanol solution or the like.

[0092] Furthermore, in order to obtain the aforementioned high strength in the non-oriented electromagnetic steel sheet of the present invention, in addition to refining the grains, it is preferable that when observing the sheet thickness section parallel to the rolling direction, the non-recrystallized structure remains at 1% or more in terms of area ratio. More preferably, it is in the range of 10% to 42%. It should be noted that the average grain size of the ferrite when the non-recrystallized structure remains refers only to the average grain size of the recrystallized grains.

[0093] σ S : Above 2.0MPa, σ C : 2.0MPa or more

[0094] Furthermore, in the non-oriented electromagnetic steel sheet of the present invention, the compressive residual stress σ in the width direction of the steel sheet surface is determined using X-ray stress measurement. S and the compressive residual stress σ at the center of the plate thickness C All values ​​need to be above 2.0 MPa. σ is preferred. S : Above 5MPa, σ C : Above 5 MPa. Here, the above X-ray stress measurement method uses the 2θ-sin α-Fe(211) peak.2 The compressive stress measured by the ψ method is a value calculated based on the lattice spacing of the {211} plane of Fe. The compressive stress detected here is calculated based on the lattice spacing of the {211} plane of Fe, while tensile stress is applied to the easy magnetization axis <100> of each grain. It should be noted that if the residual compressive stress value is greater than 100 MPa, micro-yielding occurs in some grains; therefore, the upper limit is preferably 100 MPa.

[0095] It should be noted that the residual stress introduced in this invention needs to be approximately uniform in the thickness direction. This is because if the residual stress varies in the thickness direction, the effects of this invention will not be achieved. For example, when shot peening applies compressive stress to the surface of a steel plate, tensile stress is generated in the center of the plate thickness, thus failing to achieve the effects of this invention.

[0096] Next, the manufacturing method of the non-oriented electromagnetic steel sheet of the present invention will be described.

[0097] The steel billet (slab) used in the manufacture of the non-oriented electromagnetic steel sheet of the present invention can be adjusted to the above-mentioned composition by secondary refining of molten steel obtained by vacuum degassing treatment or the like in a converter or electric furnace, and then manufactured by continuous casting or ingot rolling.

[0098] Next, the above-mentioned slab is hot-rolled under known methods and conditions to produce a hot-rolled plate. The hot-rolled plate is then annealed as needed, pickled, and then cold-rolled once or twice or more with intermediate annealing to become a cold-rolled plate with the final plate thickness (product plate thickness).

[0099] Next, to impart the desired strength and magnetism to the cold-rolled sheet, a final annealing is performed using a continuous annealing furnace. From the viewpoint of balancing high strength and productivity, the final annealing conditions are preferably in the range of a maximum reaching temperature of less than 900°C and a soaking time of 1 to 120 seconds. More preferably, the maximum reaching temperature is in the range of 650 to 850°C and the soaking time is in the range of 5 to 30 seconds. Furthermore, from the viewpoint of suppressing oxidation, the atmosphere during final annealing is preferably a reducing atmosphere such as a dry H2-N2 mixed atmosphere.

[0100] Here, to achieve the effects of the present invention, it is crucial that the parameter ε / t, defined by the plastic elongation ε (%) in the rolling direction before and after the final annealing and the homogenization time t (s) of the final annealing, is 0.10 or higher. This is because when the parameter ε / t is less than 0.10, ε is too small to introduce sufficient residual stress into the steel sheet, or t is too large and residual stress is eliminated due to recovery. Preferably, ε / t is 0.15 or higher.

[0101] It should be noted that the plastic deformation behavior during final annealing varies depending on the steel sheet composition, final annealing conditions (annealing temperature, heating time), and line tension. Therefore, in particular, in steel sheets containing large amounts of high-temperature-strength Si and Al, ε / t can be increased by raising the annealing temperature or increasing the line tension.

[0102] Furthermore, in the aforementioned final annealing, it is important that, in order to retain the residual stress introduced into the steel sheet in the high-temperature zone until room temperature, the average cooling rate during the cooling process after the soaking treatment, from the temperature at (maximum reached temperature - 50°C) to 500°C, is at least 40°C / s. When the average cooling rate within the aforementioned temperature range is less than 40°C / s, the residual stress introduced at high temperature is released due to recovery, and the effects of the present invention are not achieved. Preferably, it is 50°C / s or more.

[0103] The annealed steel sheet is coated with an insulating film as needed to produce a product sheet. The insulating film can be any known organic, inorganic, or a combination of organic and inorganic films, without compromising the effectiveness of the invention.

[0104] Example 1

[0105] Steel containing 0.0013 wt%, 3.76 wt%, 0.52 wt%, 0.006 wt%, 0.0004 wt%, 1.32 wt%, 0.0009 wt%, 0.05 wt%, 0.04 wt%, and 0.0013 wt%, with the remainder consisting of Fe and unavoidable impurities, is smelted through a conventional refining process and continuously cast to produce steel billets (slabs). Next, the slabs are heated at 1140°C for 30 minutes and then hot-rolled to produce hot-rolled plates with a thickness of 1.8 mm. The hot-rolled plates are then annealed at 950°C for 30 seconds, pickled, and then cold-rolled to produce cold-rolled plates with a final thickness (product thickness) of 0.25 mm. Next, the cold-rolled material was subjected to final annealing in a continuous annealing furnace under the conditions shown in Table 1. At this time, the plastic elongation ε (%) before and after final annealing was measured using the method described above.

[0106] From the final annealed plate obtained in this way, a specimen with a width of 30 mm and a length of 280 mm (with the width of the plate as the length direction) was taken, and the iron loss W was determined by the Epstein test. 3 / 5k Using the above method, the eddy current loss W at 0.3T and 5kHz was calculated. e3 / 5k The value of σ was also determined using the method described above. Additionally, the residual stress σ in the width direction of the final annealed plate was measured. S and σ CFurthermore, a JIS5 specimen with the rolling direction as the tensile direction was taken from the aforementioned final annealed plate, and a tensile test was conducted according to JIS Z 2241 to determine the yield stress.

[0107] The results of the above measurements are recorded in Table 1. These results show that the non-oriented electromagnetic steel sheets manufactured under the conditions of this invention all exhibit high strength and low eddy current loss W. e3 / 5k The values ​​are low. In contrast, steel plates No. 1–4, 6, 7, and 17–20 all failed to achieve the desired residual stress and thus could not reduce eddy current losses W due to inappropriate ε / t or cooling rate during final annealing. e3 / 5k Additionally, steel plates No. 23 and 24 are examples of steel plates for which residual stress (compressive stress) is introduced to the surface of the steel plate through shot peening hardening after final annealing, σ S For compressive stress, eddy current loss W e3 / 5k Instead, it increases. In addition, since the maximum final annealing temperature of the No.26 steel plate is greater than 900°C, the yield stress is reduced to less than 500 MPa.

[0108]

[0109] Example 2

[0110] Steel with the composition shown in Table 2, consisting of Fe and unavoidable impurities, is smelted through a conventional smelting process and continuously cast to produce steel billets (slabs). Next, the slabs are heated at 1150°C for 30 minutes and then hot-rolled to produce a hot-rolled plate with a thickness of 1.8 mm. The hot-rolled plate is then annealed at 940°C for 10 seconds, pickled, and cold-rolled to produce a cold-rolled plate with a final thickness (product thickness) of 0.20 mm. Next, the cold-rolled plate is subjected to a heat treatment in a continuous annealing furnace, with a maximum reaching temperature of 750°C and a soaking time of 10 seconds. Finally, a final annealing is performed at an average cooling rate of 55°C / s from the maximum reaching temperature to 500°C. During this process, the linear tension applied to the steel plate varies within the range of 5–20 MPa, and the plastic elongation ε (%) before and after the final annealing is measured using the method described above.

[0111] From the final annealed plate obtained in this way, a specimen with a width of 30 mm and a length of 280 mm (with the width of the plate as the length direction) was taken, and the iron loss W was determined by the Epstein test. 3 / 5k Using the above method, the eddy current loss W at 0.3T and 5kHz was calculated. e3 / 5k The value of σ was also determined using the method described above. Additionally, the residual stress σ in the width direction of the final annealed plate was measured. S and σ CFurthermore, a JIS5 specimen with the rolling direction as the tensile direction was taken from the aforementioned final annealed plate, and a tensile test was conducted according to JIS Z 2241 to determine the yield stress.

[0112] The results of the above measurements are recorded in Table 2. It should be noted that even for steel plates of the same thickness, iron loss can vary greatly due to the Si and Al contents, which affect the resistivity of the steel. Therefore, eddy current loss W can be evaluated using the iron loss benchmark value W defined by the following formula. e3 / 5k The advantages and disadvantages.

[0113] W = (700 × t) / (Si + Al)

[0114] t: Plate thickness (mm), Si, Al: Their respective contents (mass %)

[0115] Based on these results, the non-oriented electromagnetic steel sheets manufactured under the conditions of this invention all exhibit high strength and low eddy current loss W. e3 / 5k It is a low value.

[0116]

[0117]

[0118]

Claims

1. A non-oriented electromagnetic steel sheet, characterized in that, It has the following composition: C: 0–0.0050% by mass, Si: 2.0–5.0% by mass, Mn: 0–3.0% by mass, P: 0–0.2% by mass, S: 0–0.0050% by mass, Al: 0–3.0% by mass, N: 0–0.0050% by mass, Cr: 0–3.0% by mass, and O: 0–0.0050% by mass, with the remainder consisting of Fe and unavoidable impurities. The ferrite has an average grain size of less than 50 μm, a yield strength of over 500 MPa, and the compressive residual stress σ in the width direction of the steel plate surface and the center of the plate thickness is measured using X-ray stress measurement. S σ C The stresses are all above 2.0 MPa, wherein the X-ray stress measurement method uses the 2θ-sin α-Fe(211) diffraction peak. 2 ψ method.

2. The non-oriented electromagnetic steel sheet according to claim 1, characterized in that, In addition to the aforementioned composition, it further contains at least one component from groups A to D below: Group A: At least one of Sn: 0–0.20% by mass and Sb: 0–0.20% by mass; Group B: At least one of Ca: 0–0.01% by mass, Mg: 0–0.01% by mass, and REM: 0–0.05% by mass; Group C: At least one of Cu: 0–0.5% by mass and Ni: 0–0.5% by mass; Group D: At least one of Ge: 0-0.05% by mass, As: 0-0.05% by mass and Co: 0-0.05% by mass.

3. The non-oriented electromagnetic steel sheet according to claim 1 or 2, characterized in that, In addition to the aforementioned composition, it further contains at least one component from groups E to I: Group E: At least one of Ti: 0–0.005% by mass, Nb: 0–0.005% by mass, V: 0–0.010% by mass, and Ta: 0–0.002% by mass; Group F: At least one of B: 0–0.002% by mass and Ga: 0–0.005% by mass; Group G: Pb: 0–0.002% by mass; Group H: Zn: 0–0.005% by mass; Group I: At least one of Mo: 0 to 0.05% by mass and W: 0 to 0.05% by mass.

4. The non-oriented electromagnetic steel sheet according to any one of claims 1 to 3, characterized in that, The non-recrystallized structure in the plate thickness section parallel to the rolling direction is more than 1% by area.

5. A method for manufacturing a non-oriented electromagnetic steel sheet, characterized in that, The method for manufacturing non-oriented electromagnetic steel sheet according to any one of claims 1 to 4 involves hot rolling, hot-rolled sheet annealing, cold rolling, and final annealing in a continuous annealing furnace of a slab having the composition described in any one of claims 1 to 3. Specifically, the maximum temperature reached during the final annealing is set to be less than 900°C. Furthermore, the average cooling rate during the final annealing cooling process, from the temperature at the highest reached temperature -50°C to 500°C, is set to be 40°C / s or higher. The parameter ε / t, defined by the plastic elongation ε in the rolling direction before and after the final annealing and the soaking time t of the final annealing, is set to 0.10 or higher. The plastic elongation ε is expressed as a percentage, and the soaking time t is expressed in seconds.

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