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

Abstract

This invention provides non-oriented electrical steel sheets with high magnetic flux density and proposes an advantageous manufacturing method thereof. When manufacturing non-oriented electrical steel sheets by heating, hot rolling, hot-rolling, cold rolling, and finish annealing a steel material containing C: 0.0050% or less, Si: 1.5~5.0%, and Mn: 3.0% or less by mass%,, the average cooling rate from the surface temperature of the cast slab after continuous casting up to 1200°C is set to 30°C / s or more, the surface temperature difference before and after slab heating before hot rolling is set to 200°C or less, and the dew point of the atmosphere during finish annealing is set to -30°C or less, thereby achieving a subscale thickness of 1.0 μm or less on the steel sheet surface and a number density N of inclusions with an equivalent circle diameter of 0.5~2.5 μm located 10 μm from the steel sheet surface. S (pcs / mm 2 The number density N of inclusions of the above size located at the center of the plate thickness C (pcs / mm 2 Make it at least 1.10 times the value of ).

Description

Technical Field

[0001] The present invention relates to an isotropic electromagnetic steel sheet having a high magnetic flux density and a method for manufacturing the same.

Background Art

[0002] An isotropic electromagnetic steel sheet is a soft magnetic material mainly used as a core material for motors and the like. In electric vehicles, drones, and the like, miniaturization of drive motors is strongly required from the viewpoints of space saving and weight reduction. On the other hand, when the motor is miniaturized, there is a problem that the output decreases. To compensate for the decrease in output accompanying miniaturization of the motor, it is effective to increase the rotation speed. On the other hand, high-speed rotation of the motor has problems such as an increase in iron loss and a decrease in the durability of the bearing, and there is a limit to increasing the rotation speed of the motor.

[0003] As a means for increasing the output other than increasing the rotation speed of the motor, increasing the magnetic flux density of the electromagnetic steel sheet, which is a core material, is effective. Conventionally, increasing the magnetic flux density of electromagnetic steel sheets by improving the grain structure has been mainly studied. For example, Patent Document 1 discloses a technique for increasing the magnetic flux density by adding Sn and P. Further, Patent Document 2 discloses a technique for increasing the magnetic flux density by controlling the crystal grain size before cold rolling and then increasing the heating rate during the heating process of the final annealing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the technology disclosed in Patent Document 1 had limitations on the amount of Sn and P that could be added, as these elements embrittle the steel. Furthermore, the technology disclosed in Patent Document 2 was only effective in improving magnetic flux density in the rolling direction, and therefore had the problem of not providing sufficient improvement for use as a core material for motors.

[0006] This invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to provide a non-oriented electrical steel sheet having a high magnetic flux density and to propose an advantageous method for manufacturing it. [Means for solving the problem]

[0007] The inventors diligently studied methods for improving magnetic flux density that differed from conventional techniques in order to solve the above problems. As a result, they found that by changing the distribution of inclusions in the thickness direction within the steel sheet, the recrystallized texture is improved and the magnetic flux density of the product sheet can be increased. Furthermore, they found that optimizing the thermal history of the slab surface temperature from continuous casting to hot rolling is effective in achieving this, leading to the development of the present invention.

[0008] In other words, the present invention relates to C: 0.0050 mass% or less, Si: 1.5~5.0 mass%, Mn: 3.0 mass% or less, P: 0.2 mass% or less, S: 0.005 mass% or less, Al: 3.0 mass% or less, N: 0.005 mass% or less, Cr: 3.0 mass% or less, Ni: 2.0 mass% or less, Cu: 1.0 mass% or less, Co: 0.2 mass% or less, Ti: 0.005 mass% or less, Nb: 0 The composition of the material contains 0.002 mass% or less, V: 0.010 mass% or less, and O: 0.005 mass% or less, with the remainder being Fe and unavoidable impurities, and the thickness of the subscale on the surface of the steel sheet is 1.0 μm or less. When the number density of inclusions with an equivalent circular diameter in the range of 0.5 to 2.5 μm that exist in a plane parallel to the surface of the steel sheet inside the steel sheet is defined as the number density of inclusions at a position 10 μm from the surface of the steel sheet in the thickness direction, N S (pcs / mm 2) is the number density N of inclusions at the center of the plate thickness. C (pcs / mm 2 It is a non-oriented electrical steel sheet with a ratio of 1.10 or more.

[0009] The non-oriented electrical steel sheet of the present invention has the above-mentioned component composition, Group A; Mo:0.0010~0.050mass%, Group B; at least one selected from Sn and Sb: total of 0.001-0.20 mass%, Group C; at least one selected from Ca: 0.0001~0.01 mass%, Mg: 0.0001~0.01 mass%, and REM: 0.001~0.05 mass%, Group D; B:0.0001~0.0020mass%, Group E; Zn: 0.001~0.02 mass%, Group F; at least one selected from Pb: 0.0001~0.0020 mass%, Zr: 0.001~0.010 mass%, Ta: 0.0001~0.0020 mass%, Se: 0.0005~0.0050 mass%, Bi: 0.0001~0.0020 mass%, and W: 0.001~0.050 mass%, Group G; As: 0.001~0.020 mass%, and H group; at least one of Ge: 0.0005~0.030 mass% and Ga: 0.0005~0.030 mass% It is preferable to include at least one group of components selected from among them.

[0010] Furthermore, the present invention proposes a method for manufacturing non-oriented electrical steel sheets, comprising: manufacturing a steel material having any of the above component compositions by continuous casting; heating the steel material and then hot-rolling it to form a hot-rolled steel sheet; hot-rolled annealing the hot-rolled steel sheet to form a hot-rolled annealed sheet; cold-rolling the hot-rolled annealed sheet once or two or more times with intermediate annealing in between to form a cold-rolled steel sheet; and finishing annealing the cold-rolled steel sheet, wherein in the manufacturing of the steel material, the average cooling rate of the surface temperature in the width direction center of the cast slab during the cooling process is set to 30°C / s or more in the range of 1400°C to 1200°C; heating the steel material before hot-rolling, maintaining a temperature of 1000°C to 1200°C for 300s or more, and keeping the difference between the surface temperature of the steel material before heating and the surface temperature after heating at 200°C or less; and setting the dew point of the atmosphere at -30°C or less during the finishing annealing. Here, the surface temperature of the steel material refers to the surface temperature at the center of the long side and the center of the width of the steel material, the surface temperature of the steel material before heating refers to the lowest surface temperature of the steel material from the time of manufacture until the start of heating, and the surface temperature of the steel material after heating refers to the surface temperature of the steel material at the end of heating. [Effects of the Invention]

[0011] According to the present invention, by improving the texture of the product sheet using means different from conventional methods, it becomes possible to stably manufacture non-oriented electrical steel sheets with high magnetic flux density, which greatly contributes to increasing the output of drive motors for electric vehicles, drones, and the like. [Brief explanation of the drawing]

[0012] [Figure 1] This graph shows the effect of the number density ratio NS / NC of inclusions present in the surface layer and the center of the product plate on the magnetic flux density B50. [Figure 2] This graph shows the effect of the cooling rate of the slab surface temperature on the ratio NS / NC of the number density of inclusions in the thickness direction of the product plate. [Figure 3] This graph shows the effect of the trace component Mo content on the ratio NS / NC of the number density of inclusions in the thickness direction of the product plate. [Modes for carrying out the invention]

[0013] I will now describe the experiment that led to the development of this invention. <Experiment 1> The inventors first focused on the distribution of inclusions in the thickness direction of the product sheet as a factor influencing the magnetic properties of non-oriented electrical steel sheets, and conducted experiments to change the thermal history of the slab surface temperature from the time the slab was manufactured by continuous casting until hot rolling began. Specifically, steel having a composition containing C:0.0012 mass%, Si:3.31 mass%, Mn:0.46 mass%, P:0.01 mass%, S:0.0015 mass%, Al:0.82 mass%, N:0.0012 mass%, Cr:0.05 mass%, Ni:0.03 mass%, Cu:0.03 mass%, Co:0.01 mass%, Ti:0.0005 mass%, Nb:0.0001 mass%, V:0.0011 mass%, and O:0.0016 mass%, with the remainder being Fe and unavoidable impurities, was melted in a vacuum melting furnace. Subsequently, the molten steel was cast into a mold to form a slab-shaped steel ingot (hereinafter, this steel ingot will also be referred to as a "slab" in this experiment). In this process, the size of the mold in the slab thickness direction and the cooling method were changed, and the change in surface temperature at the center of the slab in the width direction was measured using a thermocouple installed on the inner surface of the mold. The data was compiled using the average cooling rate from 1400°C to 1200°C of the slab surface temperature at the center of the slab in the longitudinal direction and the center of the width direction.

[0014] Next, the steel ingot (slab) was removed from the mold, cooled to a surface temperature of 950°C, then placed in an electric furnace and reheated. After being held at a temperature of 1050°C for 1200 seconds, it was hot-rolled to produce a hot-rolled steel sheet with a thickness of 2.0 mm. Next, the hot-rolled steel sheet was subjected to hot-rolled annealing in an N2 atmosphere with a dew point of -40°C at an annealing temperature of 980°C for a annealing time of 20 seconds to produce a hot-rolled annealed sheet. Subsequently, the hot-rolled annealed sheet was pickled and cold-rolled to produce a cold-rolled steel sheet with a thickness of 0.25 mm. Next, the cold-rolled steel sheet was subjected to finish annealing in an atmosphere with a volume ratio of N2:H2 = 7:3 and a dew point of -40°C at an annealing temperature of 985°C for a annealing time of 10 seconds to produce a finished sheet.

[0015] Epstein test pieces were collected from the product plates thus obtained, and the magnetic flux density B 50 (magnetic flux density when the magnetizing force is 5000 A / m, unit: T) was measured in accordance with JIS C2550-1:2011. The Epstein test pieces were collected from three directions: the length direction of the test piece as the rolling direction (L direction), the direction inclined 45 degrees from the rolling direction (D direction), and the plate width direction (C direction) inclined 90 degrees from the rolling direction, and the magnetic flux density in each direction was measured.

[0016] As a result of observing the cross-section in the plate thickness direction of the above product plate by SEM, it was confirmed that no subsurface scale was observed in any of the test pieces in the surface layer part of the steel plate. Here, the above subsurface scale refers to a region where the area ratio of the second phase (oxide, nitride), such as oxide and nitride, existing in the surface layer part of the steel plate cross-section is 5% or more. Also, no subsurface scale was observed means that the thickness of the above subsurface scale is 1.0 μm or less. The size (equivalent circle diameter) of the above second phase is generally less than 0.5 μm, and the area ratio can be measured by observing the cross-section in the plate thickness direction of the surface layer part with a scanning electron microscope SEM.

[0017] Next, the surface of the test piece collected from the above product plate was removed by chemical polishing using hydrofluoric acid to expose a plane parallel to the steel plate surface at a position 10 μm in the plate thickness direction from the surface and a plane parallel to the steel plate surface at the plate thickness center position. Then, each plane was observed by SEM from the vertical direction, and the number density per unit area (number / mm 2 ) of inclusions with an equivalent circle diameter of 0.5 μm or more and 2.5 μm or less existing in each plane was measured. The number density of inclusions at a position 10 μm in the plate thickness direction from the steel plate surface was designated as N S , and the number density of inclusions at the plate thickness center position was designated as N C . The reason for limiting the size of the inclusions to be measured as above is that inclusions outside the above range have little influence on the magnetic flux density.

[0018] Regarding the above measurement results, the number density N of inclusions at the plate thickness center position CThe number density N of inclusions located 10 μm in the thickness direction from the surface of the steel plate. S The ratio (N S / N C ) and the magnetic flux density B in each direction: L direction (rolling direction), C direction (direction 90 degrees to the rolling direction), and D direction (direction 45 degrees to the rolling direction) 50 The relationships are shown in Figures 1(a) to (c), respectively. From Figures 1(a) and (b), N S / N C The value of is the magnetic flux density (B) in the L direction and C direction. 50L ,B 50C It can be seen that there is no significant impact on ). On the other hand, from Figure 1(c), the magnetic flux density B in the D direction, which has the worst characteristics, is 50D Regarding N S / N C It can be seen that the magnetic flux density can be increased by setting the value of to 1.10 or higher.

[0019] Furthermore, Figure 2 shows the average cooling rate and the number density ratio of inclusions N during the cooling of the slab surface temperature from 1400°C to 1200°C after casting. S / N C This figure shows the relationship between the number density ratio of inclusions and the cooling rate after slab casting. S / N C It can be seen that the value of can be increased. Specifically, by making the average cooling rate of the slab surface above 30°C / s or higher, the number density ratio of inclusions N S / N C It can be seen that the ratio can be increased to 1.10 or higher.

[0020] The reason for the above results is not yet fully understood, but the inventors believe it is as follows: The ratio of the number density of inclusions in the thickness direction (N S / N CA high number density indicates a high density of inclusions in the surface layer of the steel sheet. This is thought to be due to significantly increasing the cooling rate of the surface layer during slab cooling compared to the cooling rate of the center in the thickness direction. In other words, it is thought that increasing the average cooling rate of the slab surface to 30°C / s or more resulted in the deposition of oxides, nitrides, sulfides, etc., in the surface layer of the slab, increasing the number of inclusions with an equivalent circular diameter of 0.5 μm to 2.5 μm. Furthermore, by accumulating inclusions on the surface side of the slab as described above, it is thought that shear deformation is more likely to occur in the surface layer of the steel sheet with a high number density of inclusions during hot rolling and cold rolling. As a result, the recrystallized texture of the product sheet is improved, and the magnetic properties are thought to be improved.

[0021] <Experiment 2> As described above, it was found that the distribution of inclusions in the thickness direction within the steel plate affects its magnetic properties. However, even if the inclusion distribution in the slab thickness direction is optimized as described above during continuous casting, if the slab is heated to high temperatures afterward, the inclusions may grow and change in size, potentially making it impossible to maintain the inclusion distribution described above. Therefore, after manufacturing product plates under the same conditions as in <Experiment 1> described above, except that molten steel with varying amounts of the trace element Mo was produced in a vacuum melting furnace, the inclusion distribution in the thickness direction was measured in the same manner as in <Experiment 1>, and the number density ratio of the inclusions N S / N C An experiment was conducted to investigate the effect of Mo content on the value of . In this experiment, the average cooling rate from 1400°C to 1200°C of the slab surface temperature was 40°C / s, and the temperature difference before and after heating the slab was 90°C.

[0022] The above results are related to the Mo content and the number density ratio N of the intervening part. S / N C The relationship is shown in Figure 3. From this figure, Mo has a number density ratio of inclusions N S / N C It was found to have the effect of increasing N S / N CIt was found to have the effect of increasing the value of [the substance].

[0023] The reason for focusing on Mo as an additive element here is that Mo is thought to have the effect of segregating at the interface between the inclusions and the base metal, thereby suppressing the coarsening of the inclusions. In other words, by adding Mo, it becomes possible to reliably maintain the inclusion distribution in the slab thickness direction obtained by increasing the cooling rate of the slab until the finished product, thereby obtaining a steel sheet with excellent magnetic properties. This invention was developed based on the novel findings described above, with further improvements. The following describes non-oriented electrical steel sheets and methods for manufacturing the same according to embodiments of the present invention.

[0024] Next, the component composition that the non-oriented electrical steel sheet according to this embodiment and the steel material used in its manufacture should have will be described. C: 0.0050mass% or less Carbon (C) is a harmful element that causes magnetic aging, forming carbides and degrading the iron loss of the product plate. Therefore, in this embodiment, in order to suppress magnetic aging, the upper limit of the C content is set to 0.0050 mass%. Preferably, the C content is 0.003 mass% or less. Although there is no lower limit to the C content, excessive reduction of C may increase the manufacturing burden, so it is preferable to set the lower limit of the C content to around 0.0001 mass%.

[0025] Si: 1.5~5.0 mass% Si is an effective element for increasing the resistivity of steel and reducing iron loss. However, if the Si content is less than 1.5 mass%, the iron loss reduction effect is small. On the other hand, if the Si content exceeds 5.0 mass%, the steel hardens, making it difficult to roll. Therefore, the Si content should be in the range of 1.5 to 5.0 mass%. Furthermore, from the viewpoint of balancing iron loss and manufacturability, the Si content is preferably in the range of 2.5 to 4.0 mass%.

[0026] Mn:3.0mass% or less Like Si, manganese (Mn) has the effect of increasing the resistivity of steel and reducing iron loss. It also has the effect of improving hot workability. To obtain the above effects, it is preferable to add 0.2 mass% or more of Mn. On the other hand, if the Mn content exceeds 3.0 mass%, the raw material cost becomes too high, so the upper limit is set at 3.0 mass%. Furthermore, if the Mn content exceeds 2.0 mass%, the iron loss improvement effect saturates and the magnetic flux density decreases, so the preferred upper limit is 2.0 mass%.

[0027] P:0.2mass% or less P is an element used to adjust the strength of steel and can be added as appropriate. However, if the amount of P exceeds 0.2 mass%, the steel becomes brittle and rolling becomes difficult, so the upper limit for P should be 0.2 mass%. Furthermore, if P is not added for strength adjustment, it is preferable to keep it below 0.02 mass%, and if it is added, it should be in the range of 0.02 mass% to 0.10 mass%. P may inevitably be present during the steel manufacturing process. Excessive reduction of P may increase the manufacturing burden, so it is preferable to set the lower limit of P content at around 0.001 mass%.

[0028] S: 0.005mass% or less S is a harmful element that forms fine sulfides, inhibiting grain growth during hot-rolled sheet annealing and finish annealing, and reducing magnetic flux density. In particular, the above adverse effects become significant when the amount exceeds 0.005 mass%, so the upper limit is set at 0.005 mass%. Preferably, it is 0.002 mass% or less. S may be unavoidably present in the steel manufacturing process. Excessive reduction of S may increase the manufacturing burden, so it is preferable to set the S content to a lower limit of about 0.0001 mass%.

[0029] Al:3.0mass% or less Like silicon, aluminum (Al) has the effect of increasing the resistivity of steel and reducing iron loss. However, if the Al content exceeds 3.0 mass%, the steel hardens, making it difficult to roll. Therefore, the Al content should be 3.0 mass% or less. From the viewpoint of balancing iron loss and manufacturability, the Al content is preferably in the range of 0.2 to 2.0 mass%. Furthermore, reducing the Al content improves the texture and is effective in increasing the magnetic flux density. Therefore, if such effects are desired, it is preferable to reduce the Al content to 0.001 mass% or less.

[0030] N:0.005mass% or less Nitrogen (N) is a harmful element that forms fine nitrides, inhibiting grain growth during hot-rolled sheet annealing and finish annealing, and reducing magnetic flux density. In particular, these adverse effects become significant when the N content exceeds 0.005 mass%. Therefore, the upper limit for N content is set at 0.005 mass%. Preferably, the N content is 0.002 mass% or less. Since N is an element that inevitably mixes into steel as an impurity, excessive reduction may lead to increased manufacturing burden. Therefore, from a cost perspective, it is preferable to have an N content of 0.0001 mass% or more.

[0031] Cr:3.0mass% or less Cr has the effect of increasing the resistivity of steel and reducing iron loss. However, if the Cr content exceeds 3.0 mass%, it precipitates as carbonitrides, which actually worsens iron loss. Therefore, the Cr content should be 3.0 mass% or less. Furthermore, if the Cr content is less than 0.3 mass%, the above effect of reducing iron loss is small. On the other hand, if the Cr content exceeds 2.0 mass%, the iron loss after strain-relieving annealing increases. Therefore, the Cr content is preferably in the range of 0.3 to 2.0 mass%.

[0032] Ni:2.0mass% or less Ni enhances the toughness of steel and suppresses fracture during cold rolling. However, the above effect saturates when Ni is added at levels exceeding 2.0 mass%. Therefore, the Ni content should be 2.0 mass% or less. Furthermore, the toughness improvement effect is small when the Ni content is less than 0.05 mass%. On the other hand, raw material costs increase when the Ni content exceeds 0.5 mass%. Therefore, the Ni content is preferably in the range of 0.05 to 0.5 mass%.

[0033] Cu:1.0mass% or less Like nickel, copper (Cu) enhances the toughness of steel and suppresses fracture during cold rolling. However, the above effect saturates when Cu is added beyond 1.0 mass%, so the Cu content should be 1.0 mass% or less. Furthermore, the toughness improvement effect is small when the Cu content is less than 0.05 mass%. On the other hand, hot brittleness may become a problem if the Cu content exceeds 0.5 mass%. Therefore, the Cu content is preferably in the range of 0.05 to 0.5 mass%.

[0034] Co:0.2mass% or less Co has the effect of increasing the magnetic flux density of steel. However, Co is an expensive element, and raw material costs increase significantly when the Co content exceeds 0.2 mass%. Therefore, the Co content should be 0.2 mass% or less. Preferably, the Co content is in the range of 0.01 to 0.10 mass%.

[0035] Ti:0.005mass% or less Ti reacts with C and N to form carbides and nitrides at grain boundaries, thus improving strength. On the other hand, it is also a harmful element that increases the number of {111} oriented grains in the recrystallized texture and reduces magnetic flux density. In particular, the above adverse effects become significant when the Ti content exceeds 0.005 mass%. Therefore, the upper limit of the Ti content should be 0.005 mass%. Preferably, the Ti content is 0.002 mass% or less. The Ti content may be 0.

[0036] Nb:0.002mass% or less Like Ti, Nb is a harmful element that increases the strength of steel sheets, but also increases the number of {111} oriented grains in the recrystallized texture, thereby reducing the magnetic flux density. In particular, the above adverse effects become significant when the Nb content exceeds 0.002 mass%, so the upper limit should be 0.002 mass%. Preferably, it should be 0.0005 mass% or less. The Nb content may be 0.

[0037] V:0.01mass% or less V, like Ti and Nb, is a harmful element that increases the strength of steel sheets but also increases the number of {111} oriented grains in the recrystallized texture, thereby reducing the magnetic flux density. In particular, the above adverse effects become significant above 0.01 mass%, so the upper limit should be 0.01 mass%. Preferably, it should be 0.003 mass% or less. The V content may be 0.

[0038] O:0.005mass% or less O is a harmful element that forms oxide inclusions, inhibiting grain growth during hot-rolled sheet annealing and finish annealing, and reducing magnetic flux density. In particular, the above adverse effects become significant when the amount exceeds 0.005 mass%, so the upper limit should be set at 0.005 mass%. More preferably, it should be 0.002 mass% or less. O is an element that inevitably mixes into steel as an impurity, and excessive reduction may lead to an increased manufacturing burden. Therefore, from a cost standpoint, it is preferable to set the lower limit of the O content to around 0.0001 mass%.

[0039] In addition to the essential basic component composition described above, the non-oriented electrical steel sheet and the steel material used in its manufacture according to this embodiment may optionally contain the following components. Mo: 0.0010~0.05mass% Mo has the effect of suppressing the coarsening of inclusions. Therefore, by adding an appropriate amount, it is possible to more reliably maintain the thickness-direction inclusion distribution formed during the manufacturing of the steel material until the final product. To obtain the above effect, it is preferable to add 0.0010 mass% or more of Mo. However, if the Mo content exceeds 0.05 mass%, the above effect saturates, and carbides form and precipitate, adversely affecting the magnetic properties. A more preferable range is 0.01 to 0.03 mass%.

[0040] At least one selected from Sn and Sb: totaling 0.001 to 0.20 mass% Sn and Sb have the effect of improving the recrystallized texture and reducing iron loss. However, the above effect is small when the total content of Sn and Sb is less than 0.001 mass%. On the other hand, the above effect saturates when the total amount of Sn and Sb added exceeds 0.20 mass%. Therefore, it is preferable to add Sn and Sb in the range of 0.001 to 0.20 mass% in total. More preferably, the total content of Sn and Sb is in the range of 0.005 to 0.01 mass%.

[0041] At least one selected from Ca: 0.0001-0.01 mass%, Mg: 0.0001-0.01 mass%, and REM: 0.001-0.05 mass%. Ca, Mg, and REM form large, stable sulfides with an equivalent circle diameter exceeding 2.5 μm within the steel sheet, in a plane parallel to the steel sheet surface. This reduces the fine sulfides with an equivalent circle diameter of around 0.1 μm that adversely affect magnetic properties. Therefore, it has the effect of improving grain growth during hot-rolled sheet annealing and finish annealing, thereby increasing magnetic flux density. However, the above effect cannot be fully obtained with content below the lower limit. On the other hand, if added above the upper limit, the magnetic flux density will actually decrease. Therefore, it is preferable to add within the above range. More preferably, the range is Ca: 0.001~0.005 mass%, Mg: 0.0005~0.003 mass%, and REM: 0.005~0.03 mass%.

[0042] B: 0.0001~0.0020 mass% B forms stable nitrides, reducing the amount of fine nitrides with an equivalent circle diameter of less than 0.5 μm that exist within the steel sheet in a plane parallel to the steel sheet surface. Therefore, it has the effect of improving grain growth during hot-rolled sheet annealing and finish annealing, thereby increasing magnetic flux density. However, the above effect cannot be sufficiently obtained if the B content is less than 0.0001 mass%. On the other hand, if B is added in amounts exceeding 0.0020 mass%, the nitrides inhibit grain growth during hot-rolled sheet annealing, causing the pre-cold-rolled structure to become finer-grained, and conversely, the magnetic flux density decreases. Therefore, when adding B, it is preferable to have a B content in the range of 0.0001 to 0.0020 mass%. More preferably, the B content is in the range of 0.0003 to 0.0010 mass%.

[0043] Zn: 0.001~0.02 mass% Zn forms large, stable sulfides or oxides with an equivalent circular diameter exceeding 2.5 μm within the steel sheet, in a plane parallel to the steel sheet surface. This improves grain growth during hot-rolled sheet annealing and finish annealing, thereby increasing magnetic flux density. However, this effect is not sufficiently obtained with a content below the lower limit, while adding more than the upper limit actually decreases the magnetic flux density. Therefore, it is preferable to add Zn within the above range. More preferably, the Zn content is in the range of 0.002 to 0.005 mass%.

[0044] At least one selected from Pb: 0.0001~0.0020 mass%, Zr: 0.001~0.010 mass%, Ta: 0.0001~0.0020 mass%, Se: 0.0005~0.0050 mass%, Bi: 0.0001~0.0020 mass%, and W: 0.001~0.050 mass%. The above elements are all effective in improving the workability and increasing the strength of steel, and can be added as appropriate. However, the above effects cannot be sufficiently obtained if the content is below the above lower limit. On the other hand, adding more than the above upper limit increases iron loss. Therefore, it is preferable to add them within the above range. More preferably, the ranges are Pb: 0.0002~0.0010 mass%, Zr: 0.002~0.005 mass%, Ta: 0.0002~0.0010 mass%, Se: 0.0010~0.0030 mass%, Bi: 0.0002~0.0010 mass%, and W: 0.002~0.020 mass%.

[0045] As: 0.001~0.020 mass% As is a grain boundary segregation element and has the effect of reducing iron loss by improving the texture. The above effect can be obtained by adding 0.001 mass% or more of As. However, As is also an element that causes grain boundary embrittlement. The above-mentioned drawbacks become particularly noticeable when the As content exceeds 0.020 mass%. Therefore, when adding As, it is preferable to keep the As content in the range of 0.001 to 0.020 mass%. More preferably, the As content is in the range of 0.002 to 0.005 mass%.

[0046] At least one of the following: Ge: 0.0005~0.030 mass% and Ga: 0.0005~0.030 mass% Ge and Ga are both elements that improve texture. To reliably obtain the above effect, it is preferable to add at least 0.0005 mass% of Ge and Ga, respectively. On the other hand, the above effect saturates even if Ge and Ga are added beyond 0.030 mass% each. Therefore, it is preferable to set the upper limit of the Ge content and Ga content at 0.030 mass% each. More preferably, the Ge content and Ga content are in the range of 0.003 to 0.010 mass% each.

[0047] In the non-oriented electrical steel sheet and the steel material used in its manufacturing method according to this embodiment, the remainder of the components other than those described above consists of Fe and unavoidable impurities. As for unavoidable impurities, the content below the lower limit of the above-mentioned optional elements is acceptable as long as it does not diminish the effects of the present invention.

[0048] Next, a method for manufacturing non-oriented electrical steel sheets according to this embodiment will be described. The method for producing molten steel having the component composition described above is not particularly limited and can be used with converters, electric furnaces, ladle smelting furnaces, vacuum degassing equipment, and other known devices and methods. Furthermore, the method of producing molten steel in an electric furnace using scrap or reduced iron as raw materials is advantageous from the viewpoint of reducing CO2 emissions. Next, the molten steel is used, for example, as a slab to be used as steel material by a continuous casting method. While the thickness of the slab is not specifically defined, it is preferable to have a thickness in the range of 50 to 250 mm from the viewpoint of manufacturability.

[0049] In the continuous casting process described above, molten steel is poured into a water-cooled mold and cooled by the mold to form a solidified shell on the contact surface between the mold and the molten steel, thereby creating a cast slab. Furthermore, cooling water is sprayed onto the surface of the cast slab after it has been removed from the mold to further increase the thickness of the solidified shell, thereby producing a slab. Here, what is important to obtain the effects of the present invention is to control the slab cooling rate from continuous casting up to 1200°C. Specifically, during the cooling process of the cast slab, it is necessary to set the average cooling rate from 1400°C to 1200°C at the center of the slab's width direction to 30°C / s or more. By increasing the cooling rate of the slab surface, inclusions present near the slab surface (surface layer) become finer, and the distribution of inclusions in the slab thickness direction changes. In other words, the number density of inclusions with an equivalent spherical diameter of 0.5 μm to 2.5 μm present in the slab surface layer increases compared to the number density of inclusions of the same size present in the center of the slab's thickness direction. As a result, the ratio of inclusion number densities between the surface layer and the center of the steel sheet thickness can be set to a desired value. Specifically, the inclusion number density is defined as the number density of inclusions with an equivalent circular diameter in the range of 0.5 to 2.5 μm that exist in a plane parallel to the surface of the steel sheet. Then, the inclusion number density N is defined as the number of inclusions at a position 10 μm from the surface of the steel sheet in the thickness direction. S (pcs / mm 2 ) is the number density N of inclusions at the center of the plate thickness. C (pcs / mm 2 An inclusion distribution of 1.10 times or more can be obtained. During the cooling process of the cast slab, the average cooling rate from 1400°C to 1200°C at the center of the width direction of the cast slab is preferably 50 to 100°C / s. However, in continuous casting with a cast slab thickness of 10 mm or less, such as strip casting, a very high cooling rate can be obtained. However, since there is no difference in the cooling rate in the thickness direction, the above effect of the present invention cannot be obtained.

[0050] While no specific cooling conditions are required for slab surface temperatures below 1200°C, it is preferable to cool the slab to a temperature range of 800°C to 1100°C at a cooling rate of 1°C / s or higher to ensure productivity.

[0051] After continuous casting, the slabs, cooled to the above temperature range, are then transferred to the hot rolling process, charged into a heating furnace, heated to a predetermined temperature, held for a predetermined time, and then subjected to hot rolling. In this embodiment, it is necessary to transport the slabs to the heating furnace of the hot rolling process while maintaining a high temperature after continuous casting. Therefore, it is preferable to immediately transport the slabs, which have been cut with a gas torch or the like after continuous casting, to the heating furnace of the hot rolling process. Furthermore, using equipment that directly connects continuous casting and hot rolling, such as a thin slab caster, is preferable because it allows the steel material to be immediately transported to the heating process without cutting.

[0052] The purpose of the slab heating described above is to equalize the temperature distribution within the slab after continuous casting and cooling, thereby equalizing the rolling load during hot rolling and stabilizing the shape of the steel sheet. Additionally, it aims to promote the growth of fine precipitates with an equivalent spherical diameter of 0.1 μm or less, which inhibit grain growth during the subsequent annealing process, by coarsening them to inclusions with an equivalent spherical diameter exceeding 2.5 μm, thus rendering them harmless. To achieve this, the slab heating must be performed under conditions of heating and holding the slab surface at a temperature of 1000-1200°C for 300 seconds or more. If the slab heating temperature is below 1000°C or the holding time is less than 300 seconds, the above effects will not be obtained. On the other hand, if the temperature exceeds 1200°C, some sulfides and nitrides will not precipitate, and the amount of precipitates with an equivalent spherical diameter of 0.1 μm or less will increase in subsequent processes, leading to a deterioration of magnetic properties. While there is no specific upper limit for the holding time during slab heating, it is preferable to keep it below 120 minutes from the viewpoint of reducing thermal energy costs.

[0053] Here, the important point is that when heating the slab in the above heating furnace, the difference in slab surface temperature before and after heating must be kept below 200°C. Generally, slabs are heated by inputting heat from the slab surface using an electric furnace or direct-fired furnace, so the surface side of the slab tends to be hotter and the central side in the thickness direction tends to be colder. As a result, fine inclusions present on the slab surface undergo Ostwald growth, reducing the number of inclusions between 0.5 μm and 2.5 μm. In other words, the number density of inclusions in the surface layer inside the steel plate N SThis will result in a decrease in the number density distribution of inclusions in the slab thickness direction formed during slab manufacturing. However, by keeping the difference in slab surface temperature before and after slab heating to 200°C or less, the change in the difference in the number density of inclusions described above can be suppressed to the point where it can be almost ignored. A more preferable temperature difference is 100°C or less. Here, the slab surface temperature refers to the surface temperature at the center of the long side and the center of the width direction of a roughly rectangular slab. The slab surface temperature before slab heating refers to the lowest value of the slab surface temperature from the end of continuous casting until the start of slab heating. The slab surface temperature after slab heating refers to the slab surface temperature at the end of slab heating.

[0054] The slab heated under the above conditions is then hot-rolled to form a hot-rolled steel sheet of a predetermined thickness. The hot rolling can usually be carried out under known conditions and is not particularly limited. For example, from the viewpoint of stabilizing the shape of the steel sheet and suppressing oxidation and nitriding of the steel sheet surface, it is preferable that the finishing temperature of the hot rolling be 900°C or less and the coiling temperature be 700°C or less. The finishing thickness of the hot-rolled sheet is also not particularly limited, but a range of 0.5 to 4 mm is preferred.

[0055] Next, the hot-rolled steel sheet obtained as described above is subjected to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet is then pickled and cold-rolled to obtain a cold-rolled steel sheet of the final thickness (product thickness). The purpose of the hot-rolled sheet annealing is to increase the grain size before cold rolling, thereby reducing the {111} oriented grains in the recrystallized texture of the product sheet and improving the magnetic flux density. The annealing temperature and annealing time for the hot-rolled sheet annealing are not specifically defined. When continuous annealing is performed, it is preferable that the annealing temperature is in the range of 900 to 1100°C and the annealing time is in the range of 5 to 120 s. For the subsequent cold rolling, known rolling mills such as reverse rolling mills and tandem rolling mills can be used. Note that a thinner final thickness of the cold-rolled steel sheet is preferable from the viewpoint of reducing iron loss in the product sheet. Since the manufacturing cost increases as the thickness decreases, the preferred range for the final thickness is 0.10 to 0.50 mm.

[0056] Next, the cold-rolled steel sheet with the final thickness described above is subjected to finish annealing. The purpose of the finish annealing is to impart predetermined magnetic properties to the cold-rolled steel sheet, which has a rolled structure, by causing recrystallization and grain growth. The soaking temperature and soaking time for finish annealing are not specifically defined. It is preferable that the soaking temperature for finish annealing is in the range of 900 to 1100°C and the soaking time is in the range of 5 to 120 s. For the atmosphere of finish annealing, known atmospheres such as an H2 atmosphere, an N2 atmosphere, or a mixed atmosphere of H2-N2 can be used. In finish annealing, the dew point of the atmosphere must be -30°C or lower. If the dew point is higher than -30°C, an oxide layer or nitride layer will form on the surface of the steel sheet, and the magnetic properties will deteriorate. Therefore, the effect of improving the magnetic properties may be canceled out. Here, the area ratio of the second phase (oxide, nitride) present in the cross-section of the surface layer of the steel sheet is 5% or more is defined as "subscale". In this embodiment, the thickness of the subscale from the steel plate surface is required to be 1.0 μm or less. Preferably, the thickness of the subscale from the steel plate surface is 0.1 μm or less.

[0057] Next, the steel sheet after the finish annealing described above is coated with an insulating film to form the finished product. The insulating film may be an organic film, an inorganic film, or a mixed organic-inorganic film, and any known film can be used. The insulating film may be omitted if it is not necessary for the application.

[0058] The product plate obtained as described above has the following characteristics in the distribution of inclusions in the thickness direction. Here, the number density of inclusions with an equivalent circular diameter in the range of 0.5 to 2.5 μm that exist in a plane parallel to the surface of the steel plate is defined as the number density of inclusions. The number density of inclusions at a position 10 μm from the surface of the steel plate in the thickness direction is N. S (pcs / mm 2 ) is the number density N of inclusions at the center of the plate thickness. C (pcs / mm 2 It is necessary that the number density ratio of inclusions is 1.10 times or more. S / N C When the number density ratio of inclusions N is 1.10 or higher, the desired magnetic flux density improvement effect can be obtained. Preferably, the number density ratio of inclusions N S / N C The value is 1.30 or higher.

[0059] Furthermore, if there are too few inclusions in the steel plate, the desired effect cannot be obtained. Therefore, the number density of inclusions at the center of the plate thickness N C 20 pieces / mm 2 The above is preferable. Furthermore, inclusions with an equivalent circular diameter in the range of 0.5 to 2.5 μm that exist within the steel plate in a plane parallel to the surface of the steel plate have a favorable effect on the magnetic flux density through the texture, as described above. In addition, inclusions of the above size have a weak force pinning the grain boundaries and a weak interaction with the magnetic domain walls, so they do not have a significant adverse effect on the iron loss characteristics.

[0060] The above measurement of inclusions involves grinding and removing material from the steel plate surface using known methods such as mechanical polishing or chemical polishing. Then, each surface is observed using a scanning electron microscope (SEM) from a direction perpendicular to the plate surface. The equivalent circular diameter (diameter) of the identified inclusions is calculated, and the number of inclusions within the range of 0.5 μm to 2.5 μm is counted. The observation area using the SEM is 10 mm. 2 It is preferable to keep it as described above. [Examples]

[0061] Slabs containing various components shown in Tables 1-1 and 1-2, with the remainder being Fe and unavoidable impurities, were manufactured by continuous casting. During this process, the average cooling rate when the surface temperature of the cast slab cooled from 1400°C to 1200°C was varied by adjusting the casting speed and the amount of cooling water, as shown in Tables 2-1 and 2-2. The slabs obtained by continuous casting were then heated to 1100°C in a tunnel-type electric furnace and held at this temperature for 10 minutes. At this time, the slab surface temperature at the entrance of the tunnel-type electric furnace was less than 1100°C, and the slab surface temperature at the exit was approximately 1100°C. The difference in slab surface temperatures between the entrance and exit of the electric furnace was as shown in Tables 2-1 and 2-2.

[0062] Next, the slab after heating was hot-rolled to a finishing temperature of 850°C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then wound into a coil at a temperature of 650°C. Subsequently, the hot-rolled steel sheet was subjected to hot-rolled annealing at an annealing temperature of 990°C for a duration of 20 seconds to obtain a hot-rolled annealed sheet. This hot-rolled annealed sheet was then pickled and cold-rolled in a tandem rolling mill to obtain a cold-rolled steel sheet with a final thickness of 0.30 mm. This cold-rolled steel sheet was then subjected to finish annealing at a soaking temperature of 1020°C for a duration of 10 seconds to obtain a finished product sheet. During this process, the dew point of the atmosphere during the finish annealing was varied as shown in Tables 2-1 and 2-2.

[0063] [Table 1-1]

[0064] [Table 1-2]

[0065] Next, test specimens were taken from the product plates obtained as described above and subjected to the following evaluation tests. <Measuring the thickness of the subscale> The cross-section of the above-mentioned specimen in the thickness direction was observed using SEM, and the thickness of the subscale present in the surface layer of the steel plate was measured. <Measurement of inclusion number density> For the above test specimens, the surface where 10 μm was removed in the thickness direction from the surface of the steel plate by chemical polishing with hydrofluoric acid, and the surface where the material was removed up to the center of the plate thickness, were each measured by SEM from a direction perpendicular to the plate surface at a depth of 10 mm. 2 The range was observed. The number of inclusions with an equivalent circular diameter of 0.5 to 2.5 μm was measured, and the number density of inclusions at a position 10 μm in the thickness direction from the surface was calculated as N S (pcs / mm 2 ), the number density of inclusions at the center of the plate thickness is N C (pcs / mm 2 ) and the number density ratio of the inclusions N S / N C The value of was calculated. <Measurement of magnetic properties> Test specimens measuring 30 mm in width and 280 mm in length were taken from three directions of the above-mentioned product sheet: the rolling direction (L direction), the direction inclined 45 degrees from the rolling direction (D direction), and the sheet width direction (C direction) inclined 90 degrees from the rolling direction. For each test specimen, the magnetic flux density B in each direction at a magnetization force H = 5000 (A / m) was measured using the Epstein test. 50 (B 50L, B 50C and B 50D The average magnetic flux density B in the L and C directions was measured. 50LC and magnetic flux density D in the D direction 50D Ratio B 50D / D 50LC They sought it. Furthermore, for the D direction, the magnetic flux density at a magnetization force H = 50000 (A / m) was measured and considered as the saturation magnetic flux density Bs, and B 50D We calculated / Bs. Furthermore, using test specimens in the L and C directions, the iron loss W was determined by the Epstein test at a magnetic flux density of 1.0 T and a frequency of 400 Hz. 10 / 400 (W / kg) was measured.

[0066] The results of the above evaluation tests are shown in Tables 2-1 and 2-2. From these tables, it can be seen that the steel plates manufactured under the conditions of the present invention all have high magnetic flux density in the D direction, low iron loss, and excellent magnetic properties. Specifically, in terms of magnetic flux density, B 50D / D 50LC ≥0.959, B 50D The conditions are met if / Bs ≥ 0.806 and iron loss W 10 / 400 It satisfies the requirement of ≤15.0 W / kg. Tables 2-1 and 2-2 also include the measured thickness of the subscale on the surface of the product plates. Only steel plates No. 6 and 7, where the dew point of the finishing annealing atmosphere was above -30°C, showed the formation of a subscale thicker than 1.0 μm. As a result, the magnetic flux density and iron loss were significantly deteriorated. Since the dew point of the other steel plates was below -30°C, no subscale thicker than 1.0 μm was observed in any of them. Steel plates No. 24 (designation M) and No. 27 (designation P) fractured during cold rolling, making it impossible to perform the above evaluation tests.

[0067] Table 2-1

[0068] Table 2-2

Claims

1. The composition contains C: 0.0050 mass% or less, Si: 1.5 to 5.0 mass%, Mn: 3.0 mass% or less, P: 0.2 mass% or less, S: 0.005 mass% or less, Al: 3.0 mass% or less, N: 0.005 mass% or less, Cr: 3.0 mass% or less, Ni: 2.0 mass% or less, Cu: 1.0 mass% or less, Co: 0.2 mass% or less, Ti: 0.005 mass% or less, Nb: 0.002 mass% or less, V: 0.010 mass% or less, and O: 0.005 mass% or less, with the remainder being Fe and unavoidable impurities. The thickness of the subscale on the surface of the steel plate is 1.0 μm or less. When the number density of inclusions with an equivalent circular diameter in the range of 0.5 to 2.5 μm that exist within the steel plate in a plane parallel to the steel plate surface is defined as the inclusion number density N at a position 10 μm from the steel plate surface in the thickness direction, Ｓ (pcs / mm ２ ) is the number density N of inclusions at the center of the plate thickness Ｃ (pcs / mm ２ Non-oriented electrical steel sheet, which is more than 1.10 times the size of ).

2. The aforementioned component composition is Group A; Mo: 0.0010 to 0.050 mass%, Group B; at least one selected from Sn and Sb: total of 0.001 to 0.20 mass%, Group C; At least one selected from Ca: 0.0001 to 0.01 mass%, Mg: 0.0001 to 0.01 mass%, and REM: 0.001 to 0.05 mass%. Group D; B: 0.0001 to 0.0020 mass%, Group E; Zn: 0.001 to 0.02 mass%, Group F; At least one selected from Pb: 0.0001 to 0.0020 mass%, Zr: 0.001 to 0.010 mass%, Ta: 0.0001 to 0.0020 mass%, Se: 0.0005 to 0.0050 mass%, Bi: 0.0001 to 0.0020 mass%, and W: 0.001 to 0.050 mass%, Group G; As: 0.001 to 0.020 mass%, and Group H; Ge: at least one of 0.0005 to 0.030 mass% and Ga: 0.0005 to 0.030 mass% The non-oriented electrical steel sheet according to claim 1, characterized by containing at least one group of components selected from among them.

3. A method for manufacturing a non-oriented electrical steel sheet according to claim 1 or 2, A steel material having the above-mentioned component composition is manufactured, the steel material is heated and then hot-rolled to form a hot-rolled steel sheet, the hot-rolled steel sheet is hot-rolled annealed to form a hot-rolled annealed sheet, the hot-rolled annealed sheet is cold-rolled once or cold-rolled two or more times with intermediate annealing in between to form a cold-rolled steel sheet, and the cold-rolled steel sheet is then finished annealed. In the manufacturing of the aforementioned steel material, during the cooling process of the cast slab, the average cooling rate of the surface temperature at the center of the width direction of the cast slab is set to 30°C / s or more, in the range of 1400°C to 1200°C. In the heating of the steel material before hot rolling, the temperature is maintained at 1000°C to 1200°C for 300 seconds or more, and the difference between the surface temperature of the steel material before heating and the surface temperature after heating is 200°C or less. A method for manufacturing non-oriented electrical steel sheets, characterized in that the dew point of the atmosphere is set to -30°C or lower during the finish annealing process. Here, the surface temperature of the steel material refers to the surface temperature at the center of the steel material in the long-side direction and the center of the width direction, the surface temperature of the steel material before heating refers to the lowest value of the surface temperature of the steel material from the time of manufacture until the start of heating of the steel material, and the surface temperature of the steel material after heating refers to the surface temperature of the steel material at the end of the heating of the steel material.

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