Method for producing non-oriented electrical steel sheet

Abstract

A non-oriented electrical steel sheet is produced by hot-rolling a steel slab containing, in mass%, 0.005% or less of C, 3.0-5.0% of Si, 3.0% or less of Al, 3.0% or less of Mn, and 0.01-0.5% of Cu, annealing and cold-rolling the hot-rolled sheet to form a cold-rolled sheet, and then performing final annealing, the dew point DH of the furnace atmosphere in a region from 500 DEG C to 800 DEG C in a heating zone for final annealing is set to-20 DEG C or less, and the dew point DS of the furnace atmosphere in a region from more than 800 DEG C to a soaking temperature in the heating zone and in a soaking zone is set to-40 DEG C or less; and the dew point DH and the dew point DS are controlled so as to satisfy the relationship DH > DS, and oxidation of the surface of the steel sheet after final annealing is suppressed, whereby a non-oriented electromagnetic steel sheet having low iron loss is stably obtained for a high-grade material containing a large amount of Si.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a non-oriented electromagnetic steel sheet used mainly as a core material for rotating machines. Background Art

[0002] A non-oriented electromagnetic steel sheet (silicon steel sheet) is a soft magnetic material mainly used as a core material for electric motors. In order to reduce iron loss by increasing the resistivity of steel, a large amount of Si is added as an alloy element. The non-oriented electromagnetic steel sheet is mostly used for industrial induction motors, and conventionally, a so-called low-grade material with an Si content of less than 3.0% by mass and a plate thickness of 0.5 mm to 0.35 mm has been used. Since the content of Si as a non-magnetic element in this grade is relatively low, it has the characteristic of high magnetic flux density, which is beneficial to reducing the exciting current and copper loss of the induction motor. In addition, since the plate thickness is relatively thick, for steel manufacturers, it also has the advantages of high productivity in the continuous rolling and continuous annealing production lines and can be manufactured at low cost.

[0003] On the other hand, in recent years, as a countermeasure against climate change, in order to reduce CO 2 emissions, the electrification of automobiles has been rapidly developed. An electric vehicle uses a large electric motor instead of an internal combustion engine for driving, so the electrification of automobiles requires a large amount of electromagnetic steel sheets. In addition, since the drive motor of an electric vehicle is required to be miniaturized and lightweight, as an excitation method, a strong rare earth permanent magnet is used, and in order to achieve high output, high-speed rotation is expected. Therefore, among the losses generated in the drive motor of an electric vehicle, iron loss dominates over copper loss, and for the electromagnetic steel sheet used as the core material, a more significant reduction in iron loss is strongly required.

[0004] For the above reasons, for the non-oriented electromagnetic steel sheet used in the drive motor of an electric vehicle, a high-grade material with an Si content of 3.0% by mass or more and a plate thickness of 0.3 mm or less is gradually being used instead of the conventional low-grade material. For such an electromagnetic steel sheet for electric vehicles, an extremely low iron loss value is required, so even a slight fluctuation in iron loss caused by changes in manufacturing conditions will greatly affect the product yield. Therefore, it is necessary to eliminate as much as possible the factors that cause fluctuations in iron loss.

[0005] As a technique for reducing the iron loss of a non-oriented electromagnetic steel sheet, for example, Patent Document 1 proposes the following technique: during the cooling process of the final annealing, before the temperature at the center of the plate width is cooled to 600°C, while maintaining the temperature of the entire plate width within ±20°C of the temperature at the center of the plate width, cooling is performed, thereby suppressing fluctuations in iron loss in the plate width direction.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Laid-Open No. 63-047333 Summary of the Invention

[0009] Problems to be Solved by the Invention

[0010] In the final annealing equipment for manufacturing non-oriented electrical steel sheets, a mechanism for adjusting the flow rate of the cooling gas in the plate width direction is usually provided, and the operator can adjust the gas flow rate in the plate width direction to equalize the temperature of the steel sheet. However, there is a problem that the iron loss of the non-oriented electrical steel sheet cannot be stably reduced only by the method described in Patent Document 1, that is, the technique of equalizing the temperature in the plate width direction of the steel sheet during cooling.

[0011] The present invention has been completed in view of the above problems existing in the prior art, and its object is to provide a method for stably manufacturing a non-oriented electrical steel sheet having a lower iron loss than before for high-grade materials containing a large amount of Si.

[0012] Means for Solving the Problems

[0013] In order to solve the above problems, the present inventors have repeatedly conducted in-depth studies focusing on the influence of impurities contained in the steel raw material and the final annealing conditions on the iron loss characteristics. As a result, it has been found that the reason why the iron loss of high-grade materials containing a large amount of Si cannot be sufficiently reduced in the prior art is that an oxide layer is formed on the surface of the steel sheet during the final annealing, and in order to suppress the formation of the oxide layer on the surface of the steel sheet during the above final annealing, that is, in order to suppress the oxidation of the steel sheet surface, it is important to add Cu as a steel component and appropriately control the dew point of the furnace atmosphere in the heating zone and the soaking zone during the final annealing, and thus the present invention has been developed.

[0014] The present invention based on the above-mentioned findings provides a method for manufacturing a non-oriented electrical steel sheet, wherein a steel slab is hot-rolled, the hot-rolled sheet is annealed, cold-rolled once or cold-rolled two or more times with intermediate annealing to obtain a cold-rolled sheet, and then final annealing is performed using a continuous annealing equipment having a heating zone, a soaking zone, and a cooling zone. The steel slab has a composition containing C: 0.005% by mass or less, Si: 3.0 to 5.0% by mass, Al: 3.0% by mass or less, Mn: 3.0% by mass or less, P: 0.10% by mass or less, S: 0.005% by mass or less, N: 0.005% by mass or less, Cu: 0.01 to 0.5% by mass, and O: 0.005% by mass or less, and the balance is composed of Fe and inevitable impurities. The method for manufacturing the non-oriented electrical steel sheet is characterized in that the dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the heating zone of the above final annealing ranges from 500°C to 800°C H is set to -20°C or lower, and the dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the heating zone exceeds 800°C to the soaking temperature and in the soaking zoneS Set to below -40°C, and the above dew point D H and dew point D S satisfies D H >D S and is controlled in such a manner.

[0015] The method for manufacturing the non-oriented electromagnetic steel sheet of the present invention is characterized in that the dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the cooling zone of the final annealing is from the soaking temperature to 500°C C is set to below -40°C.

[0016] In addition, the steel billet used in the method for manufacturing the non-oriented electromagnetic steel sheet of the present invention is characterized in that, on the basis of the above composition, it further contains at least one component group selected from the following groups A to J.

[0017] · Group A: At least one of Sn: 0.005 to 0.20% by mass and Sb: 0.005 to 0.20% by mass;

[0018] · Group B: At least one of Ca: 0.001 to 0.010% by mass, Mg: 0.0002 to 0.005% by mass, and REM: 0.001 to 0.05% by mass;

[0019] · Group C: Cr: 0.01 to 3.0% by mass;

[0020] · Group D: Ni: 0.01 to 1% by mass;

[0021] · Group E: At least one of Mo: 0 to 0.050% by mass and B: 0 to 0.0020% by mass;

[0022] · Group F: At least one of Ti: 0 to 0.010% by mass, Nb: 0 to 0.0050% by mass, V: 0 to 0.050% by mass, Pb: 0 to 0.0020% by mass, Zr: 0 to 0.050% by mass, Ta: 0 to 0.0020% by mass, W: 0 to 0.050% by mass, Se: 0 to 0.0050% by mass, and Bi: 0 to 0.0020% by mass;

[0023] · Group G: As: 0 to 0.020% by mass;

[0024] · Group H: Zn: 0 to 0.010% by mass;

[0025] · Group I: Co: 0 to 0.10% by mass;

[0026] · Group J: At least one of Ge: 0 to 0.030% by mass and Ga: 0 to 0.030% by mass.

[0027] In addition, the above billet used in the method for manufacturing the non-oriented electromagnetic steel sheet of the present invention is characterized in that the billet is a continuously cast billet with a thickness of 30 mm or more and 300 mm or less, which is obtained by continuously casting molten steel whose composition has been further adjusted after being tapped from a converter or an electric furnace.

[0028] Advantages of the Invention

[0029] According to the present invention, a non-oriented electromagnetic steel sheet with extremely low iron loss can be stably manufactured. Therefore, according to the present invention, a core material suitable for use in a drive motor for an electric vehicle can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a graph showing the influence of the oxygen weight per unit area on the surface of the steel sheet after final annealing on the iron loss W 17 / 200 brought about.

[0031] Figure 2 is a graph showing the influence of the dew point D of the furnace atmosphere in the heating zone of the final annealing equipment in the region from 500 °C to 800 °C H and the dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the heating zone exceeds 800 °C to the soaking temperature and in the soaking zone S on the oxygen weight per unit area on the surface of the steel sheet.

[0032] Figure 3 is a graph showing the influence of the Cu content on the oxygen weight per unit area on the surface of the steel sheet after final annealing.

[0033] Figure 4 is a graph showing the influence of the dew point D of the atmosphere in the cooling zone of the final annealing equipment C on the oxygen weight per unit area on the surface of the steel sheet after final annealing. DETAILED DESCRIPTION OF THE INVENTION

[0034] First, an experiment that was the trigger for the development of the present invention will be described.

[0035] <Experiment 1>

[0036] A steel slab having a composition containing C: 0.0015 mass%, Si: 3.32 mass%, Mn: 0.51 mass%, P: 0.009 mass%, S: 0.0012 mass%, Al: 0.63 mass%, N: 0.0015 mass%, Cu: 0.05 mass% and O: 0.0012 mass%, with the balance being composed of Fe and unavoidable impurities, is hot-rolled to produce a hot-rolled sheet with a thickness of 1.8 mm. Subsequently, the above hot-rolled sheet is subjected to hot-rolled sheet annealing at a soaking temperature of 980 °C, pickled to remove scale, and cold-rolled to produce a cold-rolled sheet with a thickness of 0.25 mm. Then, using a continuous annealing facility (final annealing facility) having a heating zone, a soaking zone, and a cooling zone, a final annealing is carried out with the heating rate between 500 °C and the soaking temperature during the heating process set to 15 °C / s, the soaking temperature set to 980 °C, and the soaking time set to 20 s. At this time, the furnace atmosphere in the heating zone and the soaking zone is set to a mixed gas of H 2 :N 2 = 20:80 by volume ratio, and the dew point of the furnace atmosphere in the soaking zone is set to -35 °C. In addition, the furnace atmosphere in the cooling zone is set to N 2 gas, and the cooling rate from the soaking temperature to 500 °C is set to 15 °C / s. It should be noted that the above heating rate and cooling rate in the final annealing of the present invention are the average heating rate and the average cooling rate, respectively (the same applies hereinafter).

[0037] The iron loss characteristics of the steel sheet after the final annealing obtained in this way were evaluated, and it was found that the iron loss value varied significantly depending on the manufacturing timing. This variation was particularly significant under the measurement conditions of high magnetic flux density and high frequency. Therefore, in the present invention, particularly, the iron loss W 17 / 200 at a maximum magnetic flux density of 1.7 T and a frequency of 200 Hz, where the variation of the iron loss value is significant, is used to evaluate the iron loss characteristics. It should be noted that the above iron loss measurement conditions correspond to a state where the exciting current of the motor is high and the rotational speed is high. In the case of an electric vehicle, for example, it corresponds to the situation of climbing a slope at a relatively high speed. When the iron loss is high, there is a problem that the temperature of the motor core rises and the rare earth magnet demagnetizes.

[0038] The inventor of the present invention analyzed the oxygen content of the steel sheet after the final annealing in order to investigate the reason for the significant variation of the iron loss W 17 / 200 due to the manufacturing timing. Then, after subtracting the oxygen content (0.0012 mass%) at the time of tapping from the obtained oxygen amount, it was converted into the oxygen amount per 1 m 2 of the steel sheet surface (both sides) (unit: g / m 2 ). It should be noted that in the present invention, this oxygen amount is referred to as "oxygen weight per unit area".

[0039] Figure 1 shows the oxygen weight per unit area and the iron loss W17 / 200 As can be seen from the figure, for steel sheets with high iron loss values, the oxygen weight per unit area becomes higher, especially when the oxygen weight per unit area exceeds 0.06 g / m 2 Therefore, the cross section of the steel sheet with a high oxygen weight per unit area was observed by SEM, and as a result, oxides of Si and Al were confirmed near the surface of the steel sheet, which were presumed to be the cause of the increased iron loss.

[0040] The present inventors consider the above phenomenon as follows. Under the conditions for measuring iron loss in a normal commercial frequency band (W 15 / 50 Even if there is a small amount of oxide layer near the surface, the area is not very excited due to the low magnetic permeability, and the impact on iron loss is small. However, under the measurement conditions with high maximum magnetic flux density, the area with low magnetic permeability near the surface of the steel plate is also excited. In addition, when the frequency becomes higher, the magnetic flux density on the surface of the steel plate is also likely to increase due to the skin effect. Therefore, for W 17 / 200 For steel, even a small amount of surface oxide can have a significant adverse effect on iron loss.

[0041] Therefore, in order to suppress the increase of iron loss, the inventors first focused on the final annealing conditions using a continuous annealing device having a heating zone, a soaking zone and a cooling zone and further repeatedly conducted research. Specifically, it is assumed that the furnace atmosphere in the heating zone and the soaking zone of the above-mentioned final annealing device is controlled separately, and experiments are conducted to confirm the influence thereof.

[0042] The experimental results are shown in the form of the effect of the dew point of the furnace atmosphere in the heating zone and the soaking zone on the oxygen weight per unit area on the steel plate surface. Figure 2 It should be noted that the dew point D H It is the dew point of the furnace atmosphere in the region where the steel plate temperature in the heating zone is from 500°C to 800°C. In addition, the dew point D S It is the dew point of the steel plate temperature in the heating zone from over 800°C to the soaking temperature and the furnace atmosphere in the soaking zone. H Below -20℃, dew point D S is below -40℃ and the above D H and D S Satisfy D H >D S The oxygen weight per unit area on the steel plate surface can be reduced to 0.06 g / m 2 It should be noted that, in the present invention, the dew point D of the furnace atmosphere in the region where the steel plate temperature in the heating zone is from 500°C to 800°C is referred to as H It is called the dew point D in the heating zone. H", the dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the heating zone ranges from above 800 °C to the soaking temperature and in the soaking zone S is referred to as "the dew point D of the soaking zone" S ".

[0043] As described above, regarding the reason why the oxygen weight per unit area of the steel sheet surface can be reduced when the relationship of the dew point D in the heating zone H > the dew point D in the soaking zone S is satisfied, the present inventors consider as follows. It is presumed that when the dew point of the atmosphere in the initial stage of heating is high, an extremely thin Si and Al oxide with high barrier properties is formed on the surface of the steel sheet, thereby suppressing oxidation in the soaking zone.

[0044] <Experiment 2>

[0045] Next, the present inventors conducted the following experiment to investigate the influence of impurities contained in the steel sheet on the oxygen weight per unit area of the steel sheet surface.

[0046] Hot-rolled steel sheets with a thickness of 1.6 mm were produced by hot-rolling steel billets having the same composition as in <Experiment 1> (except for Cu) and containing various amounts of Cu in the range of 0.003 to 0.5 mass%. The hot-rolled steel sheets were subjected to hot-rolled steel sheet annealing at a soaking temperature of 1020 °C, and then pickled to remove the scale. Next, the pickled hot-rolled steel sheets were cold-rolled to produce cold-rolled steel sheets with a thickness of 0.25 mm, and then final annealing was carried out with a heating rate between 500 °C and the soaking temperature during the heating process set to 25 °C / s, the soaking temperature set to 1010 °C, and the soaking time set to 10 s. At this time, the furnace atmosphere in the heating zone and the soaking zone during the final annealing was set to a mixed gas of H 2 : N 2 = 30:70 by volume ratio, the dew point D in the heating zone H was set to -35 °C, and the dew point D in the soaking zone S was set to -50 °C. In addition, the furnace atmosphere in the cooling zone was set to N 2 gas, and the cooling rate from the soaking temperature to 500 °C was set to -30 °C / s.

[0047] Next, sample materials were cut from the steel sheets after the above final annealing, and the oxygen weight per unit area of the steel sheet surface was measured in the same manner as in <Experiment 1>. Figure 3 shows the influence of the Cu content on the oxygen weight per unit area of the steel sheet surface. From this figure, it can be seen that by adding 0.01 mass% or more of Cu, oxidation of the steel sheet surface during the final annealing can be significantly suppressed.

[0048] Although its mechanism is not yet fully understood, it is considered that Cu is an element that is more difficult to oxidize than Fe. Therefore, it is enriched on the surface of the steel substrate through atmospheric oxidation and pickling, and plays a role in promoting the formation of an oxide film with high barrier properties during final annealing. It should be noted that although Figure 3 does not show the results of Cu: 0.10 to 0.50 mass%, within the above range, the oxygen weight per unit area does not change significantly, and is about 0.005 g / m 2 .

[0049] <Experiment 3>

[0050] Next, the present inventors conducted the following experiment to investigate the effect of the dew point of the furnace atmosphere in the cooling zone of the final annealing on the oxygen weight per unit area of the steel plate surface after the final annealing.

[0051] A steel slab having a composition containing C: 0.0011 mass%, Si: 3.54 mass%, Mn: 0.56 mass%, P: 0.005 mass%, S: 0.0009 mass%, Al: 0.81 mass%, N: 0.0012 mass%, Cu: 0.05 mass% and O: 0.0006 mass%, and the balance being composed of Fe and unavoidable impurities was hot-rolled to produce a hot-rolled plate with a thickness of 1.7 mm. Next, the above hot-rolled plate was subjected to hot-rolled plate annealing at a soaking temperature of 900 °C, pickled to remove scale, and then cold-rolled to produce a cold-rolled plate with a thickness of 0.25 mm. Next, the above cold-rolled plate was subjected to a final annealing in which the heating rate from 500 °C to the soaking temperature during the heating process was set to 30 °C / s, the soaking temperature was set to 1000 °C, and the soaking time was set to 5 s. At this time, the atmospheres in the heating zone and soaking zone of the final annealing were set to a mixed gas of H 2 :N 2 = 20:80, the dew point D H in the heating zone was set to -35 °C, and the dew point D S in the soaking zone was set to -54 °C. In addition, the atmosphere in the cooling zone was set to N 2 gas, the cooling rate from the soaking temperature to 500 °C was set to 15 °C / s, and the dew point D C of the furnace atmosphere in the above cooling zone was varied within the range of 0 °C to -70 °C.

[0052] Next, a sample material was cut from the steel plate after the above final annealing, and the oxygen weight per unit area of the steel plate surface was measured in the same manner as in <Experiment 1>, and the results are shown in Figure 4 . It can be seen from this figure that when the dew point D C of the furnace atmosphere in the cooling zone exceeds -40 °C, the oxygen weight per unit area increases sharply. By setting the above dew point D CControlled below -40°C, it is possible to further reduce the oxygen weight per unit area on the steel plate surface.

[0053] The present invention has been developed based on the above insights through further repeated research.

[0054] Next, the composition of the steel raw material (slab) used to manufacture the non-oriented electrical steel sheet of the present invention will be described.

[0055] C: 0.005 mass% or less

[0056] When a large amount of C is contained in the product sheet, it becomes a cause of magnetic aging. Due to the long-term use of the motor at high temperature, carbides precipitate, which becomes a cause of increased iron loss. Therefore, C is limited to 0.005 mass% or less. Preferably, it is 0.003 mass% or less.

[0057] Si: 3.0 - 5.0 mass%

[0058] Si is an element that increases the resistivity of steel and reduces iron loss. In the drive motor for electric vehicles, extremely low iron loss is required, so it is necessary to add 3.0 mass% or more. However, when it exceeds 5.0 mass%, cold rolling becomes significantly difficult. Therefore, the upper limit is set to 5.0 mass%. The preferred range of Si is 3.5 - 4.5 mass%.

[0059] Mn: 3.0 mass% or less

[0060] Mn is an element that improves hot workability and is also an element that increases the resistivity of steel and reduces iron loss. Therefore, it can be added as needed. Preferably, it is 0.01 mass% or more. However, when it exceeds 3.0 mass%, instead, the magnetic flux density decreases or the iron loss deteriorates. Therefore, the upper limit is set to 3.0 mass%. The preferred range of Mn is 0.3 - 2.0 mass%.

[0061] P: 0.10 mass% or less

[0062] P is an element that hardens and embrittles steel. Especially when it exceeds 0.10 mass%, cold rolling becomes difficult. Therefore, in the present invention, the content of P is limited to 0.10 mass% or less. Preferably, it is 0.03 mass% or less. It should be noted that when P is used to increase the strength of steel and improve blanking workability, it is preferably added in the range of 0.03 - 0.07 mass%.

[0063] S: 0.005 mass% or less

[0064] S is a harmful element that forms sulfides, hinders grain growth, and increases iron loss. Especially when it exceeds 0.005 mass%, the above effects become significant. Therefore, it is limited to 0.005 mass% or less. Preferably, it is 0.003 mass% or less.

[0065] Al: 3.0 mass% or less

[0066] Al, like Si, is an element that increases the resistivity of steel and reduces iron loss, so it can be added as needed. However, when it exceeds 3.0 mass%, cold rolling becomes significantly difficult, so the upper limit is set at 3.0 mass%. Preferably, it is in the range of 0.3 - 2.0 mass%.

[0067] N: 0.005 mass% or less

[0068] N is a harmful element that forms nitrides, hinders grain growth, and increases iron loss. Especially when it exceeds 0.005 mass%, the above effects become significant, so it is limited to 0.005 mass% or less. Preferably, it is 0.003 mass% or less.

[0069] Cu: 0.01 - 0.5 mass%

[0070] Cu has the effect of suppressing oxidation on the steel plate surface during final annealing and suppressing the increase in iron loss. The above effect is obtained by adding 0.01 mass% or more. However, when it exceeds 0.5 mass%, the above effect saturates, the alloy cost becomes high, and thermal brittleness is likely to occur. Therefore, Cu is set in the range of 0.01 - 0.5 mass%. Preferably, it is in the range of 0.02 - 0.2 mass%.

[0071] It should be noted that Cu is one of the trace tramp elements inevitably mixed in from raw materials, and it is known that scrap steel input in the steelmaking process becomes a source of Cu. In the case of manufacturing steel billets by melting scrap steel using an electric furnace, even if Cu is not added, the Cu content becomes high, so the raw material cost can be reduced. In addition, from the perspective of reducing CO 2 emissions, the method using an electric furnace is also preferred.

[0072] O: 0.005 mass% or less

[0073] O is a harmful element that forms oxides, hinders grain growth, and increases iron loss. Especially when it exceeds 0.005 mass%, the above effects become significant, so it is limited to 0.005 mass% or less. Preferably, it is 0.003 mass% or less.

[0074] In the steel raw material used in the present invention, the balance other than the above components is substantially Fe and inevitable impurities. However, in the present invention, for the purpose of improving magnetic properties and mechanical properties, it may also contain at least one group of components selected from the following groups A - J.

[0075] Group A: At least one of Sn: 0.005 - 0.20 mass% and Sb: 0.005 - 0.20 mass%

[0076] Sn and Sb are elements effective in improving texture and reducing iron loss. The above effects are obtained by adding each at 0.005 mass% or more. However, when each exceeds 0.20 mass%, the above effects saturate. Therefore, Sn and Sb are preferably added at least one in the range of 0.005 to 0.20 mass%. More preferably, each is in the range of 0.01 to 0.10 mass%.

[0077] Group B: Ca: 0.001 to 0.010 mass%, Mg: 0.0002 to 0.005 mass%, and at least one of REM: 0.001 to 0.05 mass%

[0078] Ca, Mg, and REM are elements that improve grain growth and reduce iron loss by forming stable sulfides and reducing fine sulfides. When less than the above lower limit values, the above effects cannot be fully obtained. On the other hand, when exceeding the above upper limit values, the above effects saturate. Therefore, for Ca, Mg, and REM, it is preferable to contain at least one of Ca: 0.001 to 0.010 mass%, Mg: 0.0002 to 0.005 mass%, and REM: 0.001 to 0.05 mass%. More preferably, it is in the range of Ca: 0.002 to 0.005 mass%, Mg: 0.0005 to 0.002 mass%, and REM: 0.005 to 0.03 mass%.

[0079] Group C: Cr: 0.01 to 3.0 mass%

[0080] Cr is an element that increases the resistivity of steel and reduces iron loss. To obtain the above effects, it is preferably added at 0.01 mass% or more. On the other hand, when added exceeding 3.0 mass%, not only the iron loss deteriorates, but also the raw material cost increases. Therefore, the upper limit is preferably set at 3.0 mass%. More preferably, it is in the range of 0.03 to 2.0 mass%.

[0081] Group D: Ni: 0.01 to 1 mass%

[0082] Ni is an element that improves the toughness of steel. To obtain the above effects, it is preferably added at 0.01 mass% or more. On the other hand, when exceeding 1 mass%, not only the above effects saturate, but also the raw material cost increases. Therefore, the upper limit is set at 1 mass%. More preferably, it is in the range of 0.05 to 0.5 mass%.

[0083] Group E: at least one of Mo: 0 to 0.050 mass% and B: 0 to 0.0020 mass%

[0084] Both Mo and B have the effect of suppressing brittle fracture of steel. To reliably obtain the above effect, it is preferred to add 0.001 mass% or more of Mo and 0.0001 mass% or more of B. On the other hand, when the addition amount of Mo exceeds 0.050 mass%, carbides precipitate and the iron loss increases. In addition, when the addition amount of B exceeds 0.0020 mass%, nitrides precipitate and the iron loss increases. Therefore, the upper limit of the addition amount is preferably set to Mo: 0.050 mass%, B: 0.0020 mass%. A more preferred range is Mo: 0.010 - 0.030 mass%, B: 0.0003 - 0.0010 mass%.

[0085] Group F: at least one of Ti: 0 - 0.010 mass%, Nb: 0 - 0.0050 mass%, V: 0 - 0.050 mass%, Pb: 0 - 0.0020 mass%, Zr: 0 - 0.050 mass%, Ta: 0 - 0.0020 mass%, W: 0 - 0.050 mass%, Se: 0 - 0.0050 mass% and Bi: 0 - 0.0020 mass%

[0086] Ti, Nb, V, Pb, Zr, Ta, W, Se and Bi are all elements that improve the workability of steel or contribute to high strength, so they can be added appropriately. To reliably obtain the above effect, it is preferred to add 0.001 mass% or more of Ti, V, Zr and W respectively, and 0.0001 mass% or more of Nb, Pb, Ta, Se and Bi respectively. However, the above elements are all elements that increase the iron loss, so the upper limit is preferably set to Ti: 0.010 mass%, Nb: 0.0050 mass%, V: 0.050 mass%, Pb: 0.0020 mass%, Zr: 0.050 mass%, Ta: 0.0020 mass%, W: 0.050 mass%, Se: 0.0050 mass% and Bi: 0.0020 mass%. A more preferred range is Ti: 0.003 - 0.006 mass%, Nb: 0.0005 - 0.0030 mass%, V: 0.005 - 0.020 mass%, Pb: 0.0003 - 0.0010 mass%, Zr: 0.005 - 0.020 mass%, Ta: 0.0003 - 0.0010 mass%, W: 0.005 - 0.020 mass%, Se: 0.0005 - 0.0030 mass% and Bi: 0.0003 - 0.0010 mass%.

[0087] Group G: As: 0 - 0.020 mass%

[0088] As has the effect of increasing the hardness of steel and can be added to adjust the mechanical properties. To reliably obtain the above effect, it is preferably added in an amount of 0.001% by mass or more. However, As is also an element that embrittles steel, and particularly when it exceeds 0.020% by mass, the embrittlement becomes significant. Therefore, the upper limit is preferably set at 0.020% by mass. More preferably, it is in the range of 0.003 to 0.010% by mass.

[0089] Group H: Zn: 0 to 0.010% by mass

[0090] Zn has the effect of improving iron loss by coarsening inclusions. To reliably obtain the above effect, it is preferably added in an amount of 0.001% by mass or more. However, Zn has a high vapor pressure, and adding more than 0.010% by mass will lead to an increase in manufacturing cost. Therefore, the upper limit is preferably set at 0.010% by mass. More preferably, it is in the range of 0.002 to 0.006% by mass.

[0091] Group I: Co: 0 to 0.10% by mass

[0092] Co is an element that has the effect of increasing the magnetic flux density. To reliably obtain the above effect, it is preferably added in an amount of 0.001% by mass or more. However, Co is an expensive element, and excessive addition will lead to an increase in manufacturing cost. Therefore, the upper limit is preferably set at 0.10% by mass. A more preferable range is 0.01 to 0.05% by mass.

[0093] Group J: At least one of Ge: 0 to 0.030% by mass and Ga: 0 to 0.030% by mass

[0094] Ge and Ga have the effect of improving the texture and can therefore be added. To reliably obtain the above effect, it is preferably added in an amount of 0.001% by mass or more for each. However, even if either element is added in an amount exceeding 0.030% by mass, the above effect saturates. Therefore, the upper limit is preferably set at 0.030% by mass for each. A more preferable range for each is 0.003 to 0.010% by mass.

[0095] Next, a method for manufacturing the non-oriented electromagnetic steel sheet of the present invention will be described.

[0096] The steel raw material (slab) for manufacturing the non-oriented electromagnetic steel sheet of the present invention can be manufactured by a known method. For example, for molten steel obtained using a converter, an electric furnace, etc., secondary refining such as vacuum degassing treatment is further performed as needed. After melting the steel adjusted to the above-described composition suitable for the present invention, a slab with a thickness of 30 mm or more and 300 mm or less is formed by continuous casting. A slab with a thickness less than 30 mm is difficult to manufacture by continuous casting, and on the other hand, a slab exceeding 300 mm is difficult to hot roll. The preferred slab thickness is in the range of 100 to 250 mm.

[0097] It should be noted that, from the perspective of reducing the emissions of CO 2 , an electric furnace method that preferably uses scrap steel as the steel raw material (iron source) instead of blast furnace pig iron is adopted. In addition, when melting scrap steel in an electric furnace to produce steel billets, since the Cu contained in the scrap steel increases the Cu content, it also has the advantage of being able to reduce the Cu addition cost. It should be noted that, as the iron source, in addition to scrap steel, direct reduced iron can also be used.

[0098] Next, the above-mentioned steel billets are heated to a specified temperature and hot-rolled to produce hot-rolled sheets. Here, the heating temperature of the steel billets before the above-mentioned hot rolling is preferably set in the range of 1000 - 1200°C. When it exceeds 1200°C, a part of the precipitates dissolves and the iron loss increases. On the other hand, when it is lower than 1000°C, the deformation resistance becomes high and hot rolling becomes difficult.

[0099] In addition, the thickness of the hot-rolled sheet is preferably set in the range of 1.0 - 3.0 mm. When the thickness of the hot-rolled sheet is less than 1.0 mm, it is difficult to maintain the shape of the steel sheet after hot rolling well. On the other hand, when it exceeds 3.0 mm, cold rolling becomes difficult. In addition, a well-known thin slab continuous caster that integrates a continuous casting machine and rolling equipment can also be used for hot rolling.

[0100] Next, hot-rolled sheet annealing is performed on the above-mentioned hot-rolled sheet. The soaking temperature of the hot-rolled sheet annealing is preferably set in the range of 800 - 1100°C. When it is lower than 800°C, the recrystallization of the hot-rolled sheet becomes insufficient and the improvement effect of magnetic properties cannot be obtained sufficiently. On the other hand, when it exceeds 1100°C, the improvement effect of magnetic properties saturates. It is more preferably in the range of 900 - 1050°C.

[0101] Next, the steel sheet after the above-mentioned hot-rolled sheet annealing is descaled by pickling or the like, and then cold-rolled to produce a cold-rolled sheet with the final plate thickness (product plate thickness). The above-mentioned final plate thickness is preferably set to 0.30 mm or less. When it exceeds 0.30 mm, the eddy current loss becomes high and excellent iron loss cannot be obtained. It is more preferably 0.25 mm or less. On the other hand, the thinner the plate thickness, the lower the iron loss, but the productivity of the rolling production line and the like decreases significantly. Therefore, the lower limit of the plate thickness is preferably set to about 0.10 mm.

[0102] It should be noted that the rolling mill used in cold rolling can use well-known equipment such as a reversible rolling mill and a tandem rolling mill. In addition, from the perspective of improving magnetic properties and reducing the cold rolling load, it is set to perform cold rolling multiple times, and intermediate annealing can also be performed between them. The soaking temperature of the intermediate annealing is preferably set in the range of 900 - 1100°C. When it is lower than 900°C, the improvement effect of magnetic properties is small, and when it exceeds 1100°C, the improvement effect of magnetic properties saturates.

[0103] Next, the cold-rolled sheet with the above-mentioned final sheet thickness is subjected to final annealing using a continuous annealing apparatus (final annealing apparatus) including a heating zone, a soaking zone, and a cooling zone, and then, if necessary, an insulating coating film is applied to form a product sheet. Here, the above-mentioned final annealing is preferably performed under the conditions that the soaking temperature is 900 to 1100 °C and the soaking time is 1 to 120 s. When the soaking temperature is lower than 900 °C, grain growth is insufficient and excellent iron loss cannot be obtained. On the other hand, when it exceeds 1100 °C, not only does the iron loss reduction effect saturate, but also the thermal energy cost increases. In addition, when the soaking time is less than 1 s, it is difficult to equalize the temperature distribution in the sheet width direction. On the other hand, when it exceeds 120 s, the iron loss reduction effect saturates. In addition, the furnace atmosphere in the heating zone, soaking zone, and cooling zone of the final annealing apparatus can use a known non-oxidizing atmosphere gas. For example, H 2 gas, N 2 gas, Ar gas, CO gas, a mixed gas composed of two or more of the above gases, etc.

[0104] Here, in the present invention, it is important to appropriately control the dew point of the atmosphere in the heating zone and soaking zone of the final annealing apparatus. Specifically, it is necessary to set the dew point D H , that is, the dew point of the atmosphere in the region where the steel sheet temperature in the heating zone ranges from 500 °C to 800 °C, to -20 °C or lower, and set the dew point Ds of the soaking zone, that is, the dew point of the atmosphere in the region where the steel sheet temperature in the heating zone ranges from exceeding 800 °C to the soaking temperature and the soaking zone, to -40 °C or lower, and make the above dew point D H and D S satisfy the relationship of D H > D S . When the dew point of the atmosphere in each region exceeds the above upper limit value, the steel sheet surface oxidizes and the iron loss increases. In addition, when D H ≤ D S , a highly barrier oxide film cannot be formed during heating, and internal oxidation is promoted in the high-temperature range, so the iron loss still increases. It should be noted that when the dew point varies within each temperature region, the highest value of the dew point in each temperature region is set as D H , D S .

[0105] It should be noted that the preferred range of the dew point D H of the heating zone is -40 °C or higher and -25 °C or lower. In addition, when the dew point D H of the heating zone is within the above preferred range, the preferred range of the dew point D S of the soaking zone is -40 °C or lower. It should be noted that D H and D SThe lower limit value is not particularly specified. In a continuous annealing production line using industrial gases, it is difficult to reduce the dew point of the atmosphere to below -70°C. Therefore, it is preferably set to about -70°C.

[0106] In addition, the control of the dew point D of the in-furnace atmosphere in the cooling zone of the final annealing is also important. C The above-mentioned dew point D C is preferably controlled below -40°C. Here, the above-mentioned dew point D C is set as the dew point of the in-furnace atmosphere in the region where the steel plate temperature is cooled from the soaking temperature to 500°C. By controlling the dew point D C below -40°C, the oxygen weight per unit area on the steel plate surface can be stably reduced, so the iron loss can be further reduced. The dew point D C is more preferably below -45°C, and further preferably below -50°C. It should be noted that when the dew point varies within the above-mentioned region, the highest value of the dew point in this region is regarded as D C .

[0107] After the steel plate that has undergone the final annealing as described above, an insulating coating film is applied as needed to form a product plate. The above-mentioned insulating coating film can be any one of inorganic, organic, and a mixture of inorganic and organic, and there is no particular limitation.

[0108] Examples

[0109] For the molten steel tapped from a converter using blast furnace pig iron as the iron source, secondary refining is carried out using a vacuum degassing treatment device. After melting the steel with the component composition adjusted to contain the various components shown in Table 1 and the balance consisting of Fe and inevitable impurities, a steel slab with a thickness of 140 mm is manufactured by continuous casting. In addition, for the molten steel tapped from an electric furnace using scrap steel as the iron source, a steel slab is also manufactured in the same manner as above (No. 19 in Table 1). Then, the above-mentioned steel slab is heated at a temperature of 1100°C for 30 minutes and then hot-rolled to form a hot-rolled sheet with a thickness of 1.6 mm, and hot-rolled sheet annealing at 980°C × 30 s is carried out. Then, the steel sheet after the above-mentioned hot-rolled sheet annealing is pickled to remove the scale and then cold-rolled once to form a cold-rolled sheet with a final thickness of 0.25 mm. However, for the steel sheet of No. 18 in Table 1, after being cold-rolled to an intermediate thickness of 1.1 mm for the first time, intermediate annealing at 1100°C × 30 s is carried out, and then cold-rolled a second time to form a cold-rolled sheet with the final thickness.

[0110] Next, for the above-mentioned cold-rolled sheet, a continuous annealing equipment with a heating zone, a soaking zone, and a cooling zone is used, and the final annealing is carried out under the conditions also recorded in Table 1 to form a product plate. In the above-mentioned final annealing, the atmosphere in the heating zone and the soaking zone is set to H 2 : N 2A mixed gas of 30:70, the heating rate from 500 °C to the soaking temperature is set to 20 °C / s, heated to the soaking temperature shown in Table 1, held at the above soaking temperature for 10 s, and then cooled. The above cooling is carried out in a nitrogen atmosphere, and the cooling rate from the soaking temperature to 500 °C is set to 20 °C / s. At this time, the dew point D of the furnace atmosphere in the region from 500 °C to 800 °C in the heating zone H and the dew point D of the furnace atmosphere in the region from over 800 °C to the soaking temperature in the heating zone and in the soaking zone S and the dew point D of the furnace atmosphere in the region from the soaking temperature to 500 °C in the cooling zone C vary as shown in Table 1.

[0111] Test pieces with a width of 30 mm and a length of 280 mm with the length direction as the rolling direction are cut from the finally annealed steel sheet (product sheet) obtained in this way, and the iron loss W is measured by the Epstein test. 17 / 200 Regarding the steel sheet with an iron loss W 17 / 200 of 15.5 W / kg or less as an inventive example, and over 15.5 W / kg as a comparative example. In addition, the oxygen content of the finally annealed steel sheet (product sheet) is measured by chemical analysis, and it is converted into the oxygen weight per unit area of the steel sheet surface by the aforementioned method.

[0112] The results of the above measurements are listed in Table 1 together. From this result, it can be seen that for the steel sheets manufactured under the conditions satisfying the present invention, the oxygen weight per unit area of the steel sheet surface is all low and all show good iron loss values. It should be noted that the steel sheet of No. 20 in Table 1 is made of steel from an electric furnace, and no Cu is added particularly. Due to the Cu mixed from scrap steel, a Cu content of 0.045 mass% is obtained, and the effects of the present invention are obtained. In addition, the steel sheet of No. 19 was subjected to intermediate annealing during cold rolling, and the iron loss value was further improved due to the improvement of the texture.

[0113]

[0114]

[0115]

[0116]

Claims

1. A method for manufacturing a non-oriented electromagnetic steel sheet, wherein, a steel slab is hot-rolled, the hot-rolled sheet is annealed, and then cold-rolled once or cold-rolled two or more times with intermediate annealing to obtain a cold-rolled sheet. Subsequently, final annealing is performed using a continuous annealing apparatus having a heating zone, a soaking zone, and a cooling zone. The steel slab has a composition containing C: 0.005% by mass or less, Si: 3.0 - 5.0% by mass, Al: 3.0% by mass or less, Mn: 3.0% by mass or less, P: 0.10% by mass or less, S: 0.005% by mass or less, N: 0.005% by mass or less, Cu: 0.01 - 0.5% by mass, and O: 0.005% by mass or less, with the balance being composed of Fe and unavoidable impurities. The method for manufacturing a non-oriented electromagnetic steel sheet is characterized in that, The dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the final annealing heating zone ranges from 500°C to 800°C H is set to -20°C or lower, and the dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the heating zone ranges from above 800°C to the soaking temperature and in the soaking zone S is set to -40°C or lower, and the dew point D H and the dew point D S satisfy the relationship of D H >D S and are controlled in such a manner.

2. The method for manufacturing a non-oriented electromagnetic steel sheet according to claim 1, characterized in that, The dew point D of the furnace atmosphere in the region where the temperature of the steel sheet in the cooling zone of the final annealing ranges from the soaking temperature to 500°C C is set to -40°C or lower.

3. The method for manufacturing a non-oriented electromagnetic steel sheet according to claim 1 or 2, characterized in that, the steel slab further contains at least one component group among the following A - J groups on the basis of the above composition, · Group A: at least one of Sn: 0.005 - 0.20% by mass and Sb: 0.005 - 0.20% by mass; · Group B: at least one of Ca: 0.001 - 0.010% by mass, Mg: 0.0002 - 0.005% by mass, and REM: 0.001 - 0.05% by mass; · Group C: Cr: 0.01 - 3.0% by mass; · Group D: Ni: 0.01 - 1% by mass; · Group E: at least one of Mo: 0 - 0.050% by mass and B: 0 - 0.0020% by mass; · Group F: at least one of Ti: 0 - 0.010% by mass, Nb: 0 - 0.0050% by mass, V: 0 - 0.050% by mass, Pb: 0 - 0.0020% by mass, Zr: 0 - 0.050% by mass, Ta: 0 - 0.0020% by mass, W: 0 - 0.050% by mass, Se: 0 - 0.0050% by mass, and Bi: 0 - 0.0020% by mass; · Group G: As: 0 - 0.020% by mass; · Group H: Zn: 0 - 0.010% by mass; · Group I: Co: 0 - 0.10% by mass; · Group J: at least one of Ge: 0 - 0.030% by mass and Ga: 0 - 0.030% by mass.

4. The method for manufacturing a non-oriented electromagnetic steel sheet according to any one of claims 1 - 3, characterized in that, the steel slab is manufactured by continuous casting of molten steel whose composition has been further adjusted after tapping from a converter or an electric furnace, and has a thickness of 30 mm or more and 300 mm or less.

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