Non-oriented electromagnetic steel sheet and method for manufacturing same

Abstract

Provided are: a non-oriented electromagnetic steel sheet having high magnetic flux density; and an advantageous method for producing the same. When producing a non-oriented electromagnetic steel sheet by subjecting a steel stock that contains, by mass, 0.0050% of C, 1.5 to 5.0% of Si, and 3.0% of Mn to heating, hot rolling, hot rolled sheet annealing, cold rolling, and finish annealing, as a result of the average cooling rate of a cast slab that has been continuously cast and has a surface temperature of up to 1,200°C being set to 30°C / s or greater, the surface temperature difference of the slab before and after heating and prior to the hot rolling being set to 200°C or less, and the dew point of the atmosphere of the finish annealing being set to -30°C, a configuration is achieved in which the thickness of the subscale of the steel sheet surface is 1.0 μm or less, and the number density NS (inclusions / mm2) of inclusions having a circle equivalent diameter of 0.5 to 2.5 μm present at positions that are 10 μm from the steel sheet surface is at least 1.10 times the number density NC (inclusions / mm2) of large inclusions present at positions at the sheet thickness center.

Description

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

[0001] The present invention relates to a non-oriented electromagnetic steel sheet having a high magnetic flux density and a method for producing the same.

[0002] Non-oriented electromagnetic steel sheets are soft magnetic materials mainly used as core materials 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, there is a problem that the output decreases when the motor is miniaturized. To compensate for the decrease in output accompanying miniaturization of the motor, it is effective to increase the rotational 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 rotational speed of the motor.

[0003] As a means for increasing the output other than increasing the rotational 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 aggregate 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 finish annealing.

[0004] JP-A-2017-066425 JP-A-2012-132070

[0005] However, in the technique disclosed in Patent Document 1 above, since Sn and P to be added are elements that embrittle steel, there is a limit to the amount to be added. Further, the technique disclosed in Patent Document 2 has a problem that it is only effective for improving the magnetic flux density in the rolling direction, and thus only an insufficient improvement effect can be obtained as a core material for motors.

[0006] The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a non-oriented electromagnetic steel sheet having a high magnetic flux density and to propose an advantageous manufacturing method therefor.

[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 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 The composition 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, the thickness of the subscale on the surface of the steel plate is 1.0 μm or less, and 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, N at a position 10 μm from the surface of the steel plate 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 It is a non-oriented electrical steel sheet with a ratio of 1.10 times or more.

[0009] The non-oriented electrical steel sheet of the present invention has the following component composition: Group A: Mo: 0.0010 to 0.050 mass%, Group B: At least one selected from Sn and Sb: 0.001 to 0.20 mass in total, 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%, It is preferable that the product contains at least one component selected from the following: Group F; 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: 0.0005 to 0.030 mass%, and Ga: 0.0005 to 0.030 mass%.

[0010] Further, the present invention manufactures a steel material having any of the above component compositions by continuous casting, heats the steel material, then performs hot rolling to obtain a hot-rolled steel sheet, subjects the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet, subjects the hot-rolled annealed sheet to one cold rolling or two or more cold rollings sandwiching an intermediate annealing to obtain a cold-rolled steel sheet, and subjects the cold-rolled steel sheet to finish annealing, which is a method for manufacturing a non-oriented electrical steel sheet. In the manufacture of the above steel material, in the cooling process of the continuous casting slab, the average cooling rate in the range of the surface temperature at the center in the width direction of the slab from 1400 °C to 1200 °C is set to 30 °C / s or more. In the heating of the steel material before the above hot rolling, it is held at a temperature of 1000 °C or more and 1200 °C or less for 300 s or more, and the difference between the surface temperature of the steel material before heating and the surface temperature after heating is set to 200 °C or less. In the above finish annealing, the dew point of the atmosphere is set to -30 °C or less. Here, the surface temperature of the above steel material refers to the surface temperature at the center in the width direction at the center of the long side direction of the steel material. The surface temperature of the steel material before heating refers to the minimum value of the surface temperature of the steel material from after the manufacture of the steel material to the start of heating the steel material. Also, the surface temperature of the steel material after heating refers to the surface temperature of the steel material at the end of heating.

[0011] According to the present invention, by improving the aggregate structure of the product sheet by means different from the conventional methods, it becomes possible to stably manufacture a non-oriented electrical steel sheet having a high magnetic flux density, which greatly contributes to increasing the output of drive motors for electric vehicles, drones, etc.

[0012] The ratio N of the number density of inclusions present in the surface layer part and the center part in the plate thickness of the product sheet Ｓ / N Ｃ is a graph showing the influence on the magnetic flux density B ５０ . The graph shows the influence of the cooling rate of the slab surface temperature on the ratio N of the number density of inclusions in the plate thickness direction of the product sheet Ｓ / N Ｃ . The graph shows the influence of the content of Mo, which is a trace component, on the ratio N of the number density of inclusions in the plate thickness direction of the product sheet Ｓ / N Ｃ .

[0013] The experiment that led to the development of this invention will now be explained. <Experiment 1> First, the inventors focused on the distribution of inclusions in the thickness direction of the product sheet as a factor that affects the magnetic properties of non-oriented electrical steel sheets, and conducted an experiment to change the thermal history of the slab surface temperature from the time the slab was manufactured by the continuous casting method until hot rolling was started. 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, molten steel was cast into a mold to form a slab-shaped steel ingot (hereafter, this steel ingot will also be referred to as the "slab" in this experiment). At this time, 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 until the slab surface temperature reached 950°C, then placed in an electric furnace and reheated. After being held at 1050°C for 1200 seconds, it was hot-rolled to produce a hot-rolled steel sheet with a thickness of 2.0 mm. Then, N2N with a dew point of -40°C was added to the hot-rolled steel sheet. ２ The hot-rolled sheet was annealed at an annealing temperature of 980°C for 20 seconds under atmospheric conditions to obtain a hot-rolled annealed sheet. Subsequently, the hot-rolled annealed sheet was pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 0.25 mm. Next, N was added to the cold-rolled steel sheet by volume. ２ : H ２ The product was prepared by performing finish annealing at a temperature of 985°C for 10 seconds in an atmosphere with a dew point of -40°C, using a ratio of 7:3.

[0015] From the product plates thus obtained, Epstein test specimens were taken and the magnetic flux density B was determined by Epstein testing in accordance with JIS C2550-1:2011.５０ The magnetic flux density (in T) at a magnetization force of 5000 A / m was measured. The Epstein specimens were sampled from three directions: the length direction of the specimen (L direction), the direction inclined 45 degrees from the rolling direction (D direction), and the width direction (C direction) inclined 90 degrees from the rolling direction. The magnetic flux density was measured in each of these directions.

[0016] Furthermore, observation of the cross-section in the thickness direction of the above-mentioned product plates using a scanning electron microscope (SEM) confirmed that no subscale was observed in the surface layer of any of the test specimens. Here, the subscale refers to an area where the area ratio of the second phase (oxides, nitrides) such as oxides and nitrides present in the surface layer of the cross-section of the steel plate is 5% or more, and the absence of subscale means that the thickness of the subscale is 1.0 μm or less. The size (equivalent circle diameter) of the second phase is generally less than 0.5 μm, and its area ratio can be measured by observing the cross-section in the thickness direction of the surface layer with a scanning electron microscope (SEM).

[0017] Next, the surface of the test piece taken from the above product plate was removed by chemical polishing with hydrofluoric acid to reveal a plane parallel to the steel plate surface at a position 10 μm from the surface in the thickness direction, and a plane parallel to the steel plate surface at the center of the thickness. Then, each plane was observed from a perpendicular direction using SEM, and the number density per unit area (pieces / mm²) of inclusions with an equivalent circular diameter of 0.5 μm or more and 2.5 μm or less present in each plane was determined. ２ The number density of inclusions at a position 10 μm from the surface of the steel plate in the thickness direction is measured and N Ｓ The number density of inclusions at the center of the plate thickness is N Ｃ The reason for limiting the size of the inclusions to be measured as described above is that inclusions outside the above range have little effect on the magnetic flux density.

[0018] Regarding the measurement results above, the number density N of inclusions at the center of the plate thickness Ｃ The number density N of inclusions located 10 μm in the thickness direction from the surface of the steel plate. Ｓ 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, can be seen. 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 the magnetic properties. However, even if the inclusion distribution in the thickness direction of the slab 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, making it impossible to maintain the inclusion distribution described above. Therefore, after manufacturing product plates under the same conditions as in <Experiment 1>, 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 [the specified value]. 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 the number density ratio N of inclusions. Ｓ / N Ｃ It was found to have the effect of increasing N. Ｓ / N Ｃ It was found to have the effect of increasing the value of [the substance].

[0023] The reason for focusing on Mo as an additive element is that Mo is thought to have the effect of segregating at the interface between 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 by further improving upon the above novel findings. The non-oriented electrical steel sheet and its manufacturing method according to embodiments of the present invention will be described below.

[0024] Next, the component composition of the non-oriented electrical steel sheet and the steel material used in its manufacture according to this embodiment will be described. C: 0.0050 mass% or less. C is a harmful element that causes magnetic aging, forming carbides and degrading the iron loss of the product sheet. 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 load, so it is preferable to set the lower limit of the C content to around 0.0001 mass%.

[0025] Si: 1.5 to 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: 3.0 mass% or less. Like Si, 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: 0.2 mass% or less. P is an element used to adjust the strength of steel and can be added as appropriate. However, if P exceeds 0.2 mass%, the steel becomes brittle and rolling becomes difficult, so the upper limit for P is set at 0.2 mass%. If P is not added for strength adjustment, it is preferable to keep it below 0.02 mass%, and if it is added, it is preferable to keep it in the range of 0.02 mass% to 0.10 mass%. P may inevitably be present in steel during the 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: 0.005 mass% 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 contained in the steel manufacturing process. Excessive reduction of S may increase the manufacturing burden, so it is preferable to set the lower limit of S content to around 0.0001 mass%.

[0029] Al: 3.0 mass% or less. Like Si, 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 Al improves the texture and is effective in increasing magnetic flux density. Therefore, if such effects are desired, it is preferable to reduce it to 0.001 mass% or less.

[0030] N: 0.005 mass% or less. 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, the above adverse effects become significant when the N content exceeds 0.005 mass%. Therefore, the upper limit of the N content is set at 0.005 mass%. Preferably, the N content is 0.002 mass% or less. N is an element that inevitably mixes into steel as an impurity, and excessive reduction may lead to an increase in manufacturing burden. Therefore, from a cost standpoint, it is preferable to have an N content of 0.0001 mass% or more.

[0031] Cr: 3.0 mass% 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. Note that 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: 2.0 mass% or less. Ni has the effect of increasing the toughness of steel and suppressing fracture during cold rolling. However, the above effect saturates when Ni is added in amounts exceeding 2.0 mass%. Therefore, the Ni content should be 2.0 mass% or less. Note that 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: 1.0 mass% or less. Like Ni, Cu has the effect of increasing the toughness of steel and suppressing fracture during cold rolling. However, the above effect saturates even if Cu is added in amounts exceeding 1.0 mass%, so the Cu content should be 1.0 mass% or less. Note that the toughness improvement effect is small when the Cu content is less than 0.05 mass%. On the other hand, if the Cu content exceeds 0.5 mass%, hot brittleness may become a problem. Therefore, the Cu content is preferably in the range of 0.05 to 0.5 mass%.

[0034] Co: 0.2 mass% or less. Co has the effect of increasing the magnetic flux density of steel. However, Co is an expensive element, and if the Co content exceeds 0.2 mass%, the raw material cost increases significantly. 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: 0.005 mass% 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 is set to 0.005 mass%. Preferably, the Ti content is 0.002 mass% or less. The Ti content may be 0.

[0036] Nb: 0.002 mass% or less. Nb, like Ti, has the effect of increasing the strength of steel sheets, but it is also a harmful element that increases the {111} orientation grains in the recrystallized texture and decreases the magnetic flux density. In particular, the above adverse effects become significant when the Nb content exceeds 0.002 mass%, so the upper limit is set at 0.002 mass%. Preferably, it is 0.0005 mass% or less. The Nb content may be 0.

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

[0038] O: 0.005 mass% 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 is set at 0.005 mass%. More preferably, it is 0.002 mass% or less. O is an element that inevitably mixes into steel as an impurity, and excessive reduction may lead to an increase in manufacturing burden. Therefore, from the viewpoint of cost, it is preferable to set the lower limit of the O content to about 0.0001 mass%.

[0039] The non-oriented electrical steel sheet and the steel material used in its manufacture according to this embodiment may optionally contain the following components in addition to the above-mentioned essential basic component composition: Mo: 0.0010 to 0.05 mass% Mo has the effect of suppressing the coarsening of inclusions. Therefore, by adding an appropriate amount, it is possible to more reliably maintain the distribution of inclusions in the thickness direction formed during the manufacture 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 will saturate, and carbides will 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: total of 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 of the Sn and Sb content is in the range of 0.005 to 0.01 mass%.

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

[0042] B: 0.0001 to 0.0020 mass% B forms stable nitrides, reducing 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 in hot-rolled sheet annealing and finish annealing and 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 in hot-rolled sheet annealing, causing the pre-cold-rolled structure to become finer, 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 to 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 of the above elements, 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%, is 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 lower limit. On the other hand, adding more than the upper limit increases iron loss. Therefore, it is preferable to add within the above range. More preferably, the ranges are Pb: 0.0002 to 0.0010 mass%, Zr: 0.002 to 0.005 mass%, Ta: 0.0002 to 0.0010 mass%, Se: 0.0010 to 0.0030 mass%, Bi: 0.0002 to 0.0010 mass%, and W: 0.002 to 0.020 mass%.

[0045] As: 0.001 to 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 can cause 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] Ge: 0.0005 to 0.030 mass% and Ga: 0.0005 to 0.030 mass%. Both Ge and Ga are elements that improve texture. To reliably obtain the above effect, it is preferable to add 0.0005 mass% or more of Ge and Ga, respectively. On the other hand, if Ge and Ga are added in amounts exceeding 0.030 mass%, the above effect will saturate. Therefore, it is preferable to set the upper limits of the Ge content and Ga content to 0.030 mass%, respectively. More preferably, the Ge content and Ga content are in the range of 0.003 to 0.010 mass%, respectively.

[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 manufacturing molten steel having the component composition described above is not particularly limited and can be manufactured using a converter, electric furnace, ladle refining furnace, vacuum degassing apparatus, and other known apparatus and methods. Note that the method of manufacturing molten steel in an electric furnace using scrap or reduced iron as raw materials is CO ２ This is advantageous from the standpoint of reducing emissions. Next, the molten steel is used, for example, as a slab to be used as steel material by a continuous casting method. The thickness of the slab is not specifically defined, but from the viewpoint of manufacturability, it is preferable to have a thickness in the range of 50 to 250 mm.

[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 slab. Furthermore, cooling water is sprayed onto the surface of the 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 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. The inclusion number density N is then defined as the inclusion number density 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. In 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 such as strip casting, where the cast slab thickness is 10 mm or less, 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 to 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 cannot 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 the 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 part in the thickness direction tends to be colder. As a result, fine inclusions present on the slab surface undergo Ostwald growth, and the number of inclusions between 0.5 μm and 2.5 μm decreases. 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 a level that 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 become 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 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. The atmosphere for finish annealing is H ２ Atmosphere, N ２ Atmosphere, H ２ -N ２ Known atmospheres, such as a mixed atmosphere, 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, degrading its magnetic properties. As a result, 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, which is defined as "subscale". In this embodiment, the thickness of the subscale from the surface of the steel sheet must be 1.0 μm or less. Preferably, the thickness of the subscale from the surface of the steel sheet 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 it be 1.10 times or more of the number density ratio of inclusions N 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 / NＣ 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 observing each surface perpendicular to the plate surface with a scanning electron microscope (SEM), calculating the equivalent circular diameter (diameter) of the identified inclusions, and counting the number of inclusions within the range of 0.5 μm to 2.5 μm. The area observed with the SEM is 10 mm. ２ It is preferable to keep it as described above.

[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 in various ways, as shown in Tables 2-1 and 2-2, by adjusting the casting speed and the amount of cooling water during continuous casting. 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 temperature 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 an annealing time of 20 s to obtain a hot-rolled annealed sheet. The 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. The cold-rolled steel sheet was then subjected to finish annealing at a soaking temperature of 1020°C for a soaking time of 10 s to obtain a finished 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]

[0064]

[0065] Next, test specimens were taken from the product plates obtained as described above and subjected to the following evaluation tests. <Measurement of subscale thickness> The thickness of the subscale present in the surface layer of the steel plate was measured by observing the cross-section of the test specimen in the thickness direction using SEM. <Measurement of inclusion number density> For the test specimen, the surface from which 10 μm of material was removed in the thickness direction from the surface of the steel plate by chemical polishing with hydrofluoric acid, and the surface from which 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 inclusions N Ｓ / N Ｃ The value was determined. <Measurement of magnetic properties> Test pieces for magnetic measurement, 30 mm wide x 280 mm long, were taken from three directions of the above product plate: the rolling direction (L direction), the direction inclined 45 degrees from the rolling direction (D direction), and the plate width direction inclined 90 degrees from the rolling direction (C direction). For each test piece, the magnetic flux density B in each direction at a magnetization force H = 5000 (A / m) in the Epstein test was determined. ５０ (B ５０Ｌ， B ５０Ｃ and B ５０ＤThe 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 Furthermore, for the D direction, the magnetic flux density at a magnetization force H = 50000 (A / m) was measured and this was considered as the saturation magnetic flux density Bs, and B ５０Ｄ The iron loss W was calculated using the Epstein test with test specimens in the L and C directions 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 together in Tables 2-1 and 2-2. From these tables, it can be seen that the steel plates manufactured to satisfy 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 for / Bs ≥ 0.806 are met, and the iron loss W is also met. １０／４００ The value ≤15.0 W / kg is satisfied. Tables 2-1 and 2-2 also include the measured thickness of the subscale on the surface of the product plate. Only steel plates No. 6 and 7, where the dew point of the finishing annealing atmosphere was above -30°C, showed subscale thickness exceeding 1.0 μm. As a result, the magnetic flux density and iron loss were significantly deteriorated. For the other steel plates, since the dew point was below -30°C, no subscale exceeding 1.0 μm was observed. Steel code M for No. 24 and steel code P for No. 27 fractured during cold rolling, making it impossible to perform the above evaluation tests.

[0067]

[0068]

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, 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 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 component composition is as follows: Group A: Mo: 0.0010 to 0.050 mass%, Group B: At least one selected from Sn and Sb: 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%, The non-oriented electrical steel sheet according to claim 1, characterized in that it contains components from 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; at least one selected from Ge: 0.0005 to 0.030 mass%, and Ga: 0.0005 to 0.030 mass%.

3. A method for manufacturing a non-oriented electrical steel sheet, comprising: manufacturing a steel material having the component composition described in claim 1 or 2; heating the steel material and then hot-rolling it to obtain a hot-rolled steel sheet; hot-rolled annealing the hot-rolled steel sheet to obtain a hot-rolled annealed sheet; cold-rolling the hot-rolled annealed sheet once or two or more times with intermediate annealing in between to obtain 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 30°C / s or more, the heating of the steel material before hot-rolling is maintained at a temperature of 1000°C or more and 1200°C or less for 300 s 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; and the dew point of the atmosphere during the finishing annealing is -30°C or less. 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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