NON-ORIENTED ELECTRICAL STEEL SHEET AND MANUFACTURING METHOD THEREOF

Abstract

A non-oriented electrical steel sheet with lower iron loss than conventional non-oriented electrical steel sheets is provided. The non-oriented electrical steel sheet has a chemical composition containing, in mass %: C: 0.05% or less; Si: 0.1% or more and 7.0% or less; Al: 0.1% or more and 3.0% or less; Mn: 0.03% or more and 3.0% or less; P: 0.2% or less; S: 0.005% or less; N: 0.005% or less; and O: 0.01% or less, and further optionally containing a predetermined amount of one or more of Sn, Sb, Ca, Mg, REM, Cr, Ti, Nb, V, and Zr, with the balance consisting of Fe and incidental impurities, wherein a sheet thickness is less than 0.30 mm, and arithmetic mean roughness Ra of a steel substrate surface at cutoff wavelength λc=20 μm is 0.2 μm or less.

Claims

1. A non-oriented electrical steel sheet having a chemical composition containing, in mass %: C: 0.05% or less; Si: 0.1% or more and 7.0% or less; Al: 0.1% or more and 3.0% or less; Mn: 0.03% or more and 3.0% or less; P: 0.2% or less; S: 0.005% or less; N: 0.005% or less; and O: 0.01% or less, with the balance consisting of Fe and incidental impurities, wherein a sheet thickness is less than 0.30 mm, and arithmetic mean roughness Ra of a steel substrate surface at cutoff wavelength λc=20 μm is 0.2 μm or less.

2. The non-oriented electrical steel sheet according to claim 1, wherein the chemical composition contains, in mass %, at least one group selected from: one or more of Sn and Sb: 0.01% or more and 0.2% or less in total, one or more of Ca, Mg, and REM: 0.0005% or more and 0.010% or less in total, Cr: 0.1% or more and 20% or less, and one or more of Ti, Nb, V, and Zr: 0.01% or more and 1.0% or less in total.

3. (canceled)

4. (canceled)

5. (canceled)

6. A manufacturing method of a non-oriented electrical steel sheet, comprising: heating a steel slab having the chemical composition according to claim 1; hot rolling the steel slab into a hot rolled steel sheet; optionally hot band annealing the hot rolled steel sheet; cold rolling the hot rolled steel sheet once or twice or more with intermediate annealing in between, into a cold rolled steel sheet whose sheet thickness is less than 0.30 mm; and final annealing the cold rolled steel sheet, wherein arithmetic mean roughness Ra of a roll surface in a final pass of last cold rolling at cutoff wavelength λc=20 μm is 0.2 μm or less.

7. A manufacturing method of a non-oriented electrical steel sheet, comprising: heating a steel slab having the chemical composition according to claim 2; hot rolling the steel slab into a hot rolled steel sheet; optionally hot band annealing the hot rolled steel sheet; cold rolling the hot rolled steel sheet once or twice or more with intermediate annealing in between, into a cold rolled steel sheet whose sheet thickness is less than 0.30 mm; and final annealing the cold rolled steel sheet, wherein arithmetic mean roughness Ra of a roll surface in a final pass of last cold rolling at cutoff wavelength λc=20 μm is 0.2 μm or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In the accompanying drawings:

[0044] FIG. 1 is a graph illustrating the relationship between the arithmetic mean roughness Ra (cutoff wavelength λc=20 μm) of the steel substrate surface and the hysteresis loss Wh.sub.10/50 in various sheet thicknesses.

DETAILED DESCRIPTION

[0045] (Non-Oriented Electrical Steel Sheet)

[0046] The following describes a non-oriented electrical steel sheet according to one of the disclosed embodiments. The reasons for limiting the chemical composition of steel are described first. In this description, “%” indicating the content of each element denotes “mass %”.

[0047] C: 0.05% or less

[0048] C can be used to strengthen the steel. When the C content exceeds 0.05%, working is difficult. The upper limit of the C content is therefore 0.05%. In the case of not using C for strengthening, the C content is preferably 0.005% or less to suppress magnetic aging.

[0049] Si: 0.1% or more and 7.0% or less

[0050] Si, when 0.1% or more is added, has an effect of increasing the specific resistance of the steel to reduce iron loss. When the Si content exceeds 7.0%, however, iron loss increases. The Si content is therefore 0.1% or more and 7.0% or less. The Si content is preferably 1.0% or more and 5.0% or less, in terms of the balance between iron loss and workability.

[0051] Al: 0.1% or more and 3.0% or less

[0052] Al, when 0.1% or more is added, has an effect of increasing the specific resistance of the steel to reduce iron loss. When the Al content exceeds 3.0%, however, casting is difficult. The Al content is therefore 0.1% or more and 3.0% or less. The Al content is preferably 0.3% or more and 1.5% or less.

[0053] Mn: 0.03% or more and 3.0% or less

[0054] Mn, when 0.03% or more is added, prevents the hot shortness of the steel. It also has an effect of increasing the specific resistance to reduce iron loss. When the Mn content exceeds 3.0%, however, iron loss increases. The Mn content is therefore 0.03% or more and 3.0% or less. The Mn content is preferably 0.1% or more and 2.0% or less.

[0055] P: 0.2% or less

[0056] P can be used to strengthen the steel. When the P content exceeds 0.2%, however, the steel becomes brittle and working is difficult. The P content is therefore 0.2% or less. The P content is preferably 0.01% or more and 0.1% or less.

[0057] S: 0.005% or less

[0058] When the S content exceeds 0.005%, precipitates such as MnS increase and grain growth degrades. The upper limit of the S content is therefore 0.005%. The S content is preferably 0.003% or less.

[0059] N: 0.005% or less

[0060] When the N content exceeds 0.005%, precipitates such as AlN increase and grain growth degrades. The upper limit of the N content is therefore 0.005%. The N content is preferably 0.003% or less.

[0061] O: 0.01% or less

[0062] When the O content exceeds 0.01%, oxides increase and grain growth degrades. The upper limit of the O content is therefore 0.01%. The O content is preferably 0.005% or less.

[0063] In addition to the aforementioned components, the following components may be added.

[0064] Sn, Sb: 0.01% or more and 0.2% or less in total

[0065] Sn and/or Sb, when 0.01% or more is added, have an effect of reducing [111] crystal grains in the recrystallization texture to improve magnetic flux density. They also have an effect of preventing nitriding and oxidation in final annealing or stress relief annealing to suppress an increase in iron loss. When the total content of Sn and/or Sb exceeds 0.2%, however, the effects saturate. The total content of Sn and/or Sb is therefore 0.01% or more and 0.2% or less. The total content of Sn and/or Sb is preferably 0.02% or more and 0.1% or less.

[0066] Ca, Mg, REM: 0.0005% or more and 0.010% or less in total

[0067] Ca, Mg, and/or REM, when 0.0005% or more is added, have an effect of coarsening sulfides to improve grain growth. When the total content of Ca, Mg, and/or REM exceeds 0.010%, however, grain growth degrades. The total content of Ca, Mg, and/or REM is therefore 0.0005% or more and 0.010% or less. The total content of Ca, Mg, and/or REM is preferably 0.001% or more and 0.005% or less.

[0068] Cr: 0.1% or more and 20% or less

[0069] Cr, when 0.1% or more is added, has an effect of increasing the specific resistance of the steel to reduce iron loss. A large amount of Cr can be added because of low steel hardness. When the Cr content exceeds 20%, however, decarburization is difficult, and carbides precipitate and cause an increase in iron loss. The Cr content is therefore 0.1% or more and 20% or less. The Cr content is preferably 1.0% or more and 10% or less.

[0070] Ti, Nb, V, Zr: 0.01% or more and 1.0% or less in total

[0071] Ti, Nb, V, and/or Zr are carbide- or nitride-forming elements. When the total content of Ti, Nb, V, and/or Zr is 0.01% or more, the strength of the steel can be enhanced. When the total content of Ti, Nb, V, and/or Zr exceeds 1.0%, however, the effect saturates. The total content of Ti, Nb, V, and/or Zr is therefore 0.01% or more and 1.0% or less. The total content of Ti, Nb, V, and/or Zr is preferably 0.1% or more and 0.5% or less. In the case of not using Ti, Nb, V, and/or Zr for strengthening, the total content of Ti, Nb, V, and/or Zr is preferably 0.005% or less to improve grain growth.

[0072] The balance other than the aforementioned elements is Fe and incidental impurities.

[0073] It is important that, in the non-oriented electrical steel sheet in this embodiment, the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm is 0.2 μm or less. By reducing fine roughness of a smaller wavelength than the domain wall displacement distance in this way, hysteresis loss can be reduced. The arithmetic mean roughness Ra is preferably 0.1 μm or less.

[0074] The measurement of the surface roughness is performed as defined in JIS B 0601, JIS B 0632, JIS B 0633, and JIS B 0651. Since the measurement is performed on the steel substrate surface, if any coating is applied to the steel substrate surface, the coating is removed by boiled alkali or the like. A measurement machine capable of accurately detecting microroughness of several μm or less in wavelength is selected to measure the surface roughness. A typical stylus-type surface roughness meter has a stylus tip radius of several μm, and so is not suitable to detect microroughness. Accordingly, a three-dimensional scanning electron microscope is used to measure the arithmetic mean roughness Ra in the disclosure. To detect microroughness, the reference length and the cutoff wavelength (cutoff value) λc are set to 20 μm. The cutoff ratio λc/λs is not particularly designated, but is desirably 100 or more. The measurement is performed with cutoff ratio λc/λs of 100 in the disclosure. The measurement directions are the rolling direction and the direction orthogonal to the rolling direction. The measurement is performed three times in each direction, and the mean value is used.

[0075] Microroughness obtained by, for example, a typical stylus-type surface roughness meter does not affect the magnetic property, and so is not particularly limited. To improve the stacking factor, it is desirable that the arithmetic mean roughness Ra of the steel substrate surface obtained at cutoff wavelength λc=0.8 mm and cutoff ratio λc/λs=300 is 0.5 μm or less.

[0076] In this embodiment, the sheet thickness is less than 0.30 mm. In the case where the sheet thickness is less than 0.30 mm, the iron loss reduction effect by limiting the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm to 0.2 μm or less is achieved. The sheet thickness is preferably 0.25 mm or less, and more preferably 0.15 mm or less. When the sheet thickness is less than 0.05 mm, the manufacturing cost increases. Accordingly, the sheet thickness is preferably 0.05 mm or more.

[0077] (Manufacturing Method of Non-Oriented Electrical Steel Sheet)

[0078] The following describes a manufacturing method of a non-oriented electrical steel sheet according to one of the disclosed embodiments. Molten steel adjusted to the aforementioned chemical composition may be formed into a steel slab by typical ingot casting and blooming or continuous casting, or a thin slab or thinner cast steel with a thickness of 100 mm or less by direct casting.

[0079] The steel slab is then heated by a typical method, and hot rolled into a hot rolled steel sheet.

[0080] The hot rolled steel sheet is then subjected to hot band annealing according to need. The hot band annealing is intended to prevent ridging or improve magnetic flux density, and may be omitted if unnecessary. A preferable condition is 900° C. to 1100° C.×1 sec to 300 sec in the case of using a continuous annealing line, and 700° C. to 900° C.×10 min to 600 min in the case of using a batch annealing line.

[0081] The hot rolled steel sheet is then pickled, and cold rolled once or twice or more with intermediate annealing in between, into a cold rolled steel sheet with the final sheet thickness. The final sheet thickness is less than 0.30 mm.

[0082] A preferable method of limiting the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm to 0.2 μm or less is to adjust the surface roughness of the rolling mill rolls in the final pass of the last cold rolling. In this embodiment, the arithmetic mean roughness Ra of the roll surface in the final pass of the last cold rolling at cutoff wavelength λc=20 μm is 0.2 μm or less. At least the final pass is preferably dry rolling, to efficiently transfer the roll surface to the steel. The surface of the cold rolled steel sheet can be smoothed in this way. In the case of not smoothing the steel substrate surface in the cold rolling, a step such as chemical polishing or electropolishing may be added after the cold rolling or final annealing, to set the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm to 0.2 μm or less. In terms of manufacturing cost, however, the steel substrate surface is preferably smoothed during the cold rolling.

[0083] After the final cold rolling, the cold rolled steel sheet is subjected to final annealing. If the steel sheet surface is oxidized or nitrided in the final annealing, the magnetic property degrades significantly. To prevent oxidation, the annealing atmosphere is preferably a reducing atmosphere. For example, it is preferable to use a N.sub.2-H.sub.2 mixed atmosphere with a H.sub.2 concentration of 5% or more, and decrease the dew point to control PH.sub.2O/PH.sub.2 to 0.05 or less. To prevent nitriding, the N.sub.2 partial pressure of the furnace atmosphere is preferably 95% or less, and more preferably 85% or less. Adding one or more of Sn and Sb in an amount of 0.01% or more and 0.2% or less in total to the steel is particularly effective in suppressing oxidation and nitriding. A preferable annealing condition is 700° C. to 1100° C.×1 sec to 300 sec. The annealing temperature may be increased in the case of placing importance on iron loss, and decreased in the case of placing importance on strength.

[0084] After the final annealing, insulating coating is applied to the steel sheet surface according to need, thus obtaining a product sheet (non-oriented electrical steel sheet). The insulating coating may be well-known coating. For example, inorganic coating, organic coating, and inorganic-organic mixed coating may be selectively used depending on purpose.

[0085] The other manufacturing conditions may comply with a typical manufacturing method of a non-oriented electrical steel sheet.

EXAMPLES

Example 1

[0086] A steel slab containing C: 0.0022%, Si: 3.25%, Al: 0.60%, Mn: 0.27%, P: 0.02%, S: 0.0018%, N: 0.0021%, O: 0.0024%, and Sn: 0.06% with the balance consisting of Fe and incidental impurities was obtained by steelmaking, heated at 1130° C. for 30 minutes, and then hot rolled into a hot rolled steel sheet. The hot rolled steel sheet was subjected to hot band annealing of 1000° C.×30 sec, and further cold rolled into a cold rolled steel sheet of 0.15 mm to 0.30 mm in sheet thickness. The obtained cold rolled steel sheet was subjected to final annealing of 1000° C.×10 sec in an atmosphere of H.sub.2:N.sub.2=30:70 with a dew point of −50° C., and then insulating coating was applied to obtain a product sheet.

[0087] Here, the microroughness of the steel substrate surface of the product sheet was changed by adjusting the surface roughness of the rolling mill rolls in the final pass of the cold rolling. Test pieces of 280 mm×30 mm were collected from the obtained product sheet, and direct-current magnetic measurement was performed by Epstein testing to measure hysteresis loss Wh.sub.10/50 with Bm=1.0 T and f=50 Hz. Moreover, after removing the insulating coating of the product sheet by boiled alkali, surface shape measurement for 100 μm×100 μm was conducted with an accelerating voltage of 5 kV using 3D-SEM (ERA-8800FE) made by Elionix Inc., and the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm was measured under the aforementioned condition. FIG. 1 illustrates the results. The results indicate that hysteresis loss was low in the disclosed range. In the case where Ra of the roll surface in the final pass of the cold rolling at cutoff wavelength λc=20 μm was 0.2 μm or less, the arithmetic mean roughness Ra of the steel substrate surface was 0.2 μm or less.

Example 2

[0088] A steel slab containing the components shown in Table 1 with the balance consisting of Fe and incidental impurities was obtained by steelmaking, heated at 1100° C. for 30 minutes, and then hot rolled into a hot rolled steel sheet. The hot rolled steel sheet was subjected to hot band annealing of 980° C.×30 sec, and further cold rolled into a cold rolled steel sheet of 0.15 mm in sheet thickness. The obtained cold rolled steel sheet was subjected to final annealing of 980° C.×10 sec in an atmosphere of H.sub.2:N.sub.2=20:80 with a dew point of −40° C., and then insulating coating was applied to obtain a product sheet.

[0089] Here, the microroughness of the steel substrate surface of the product sheet was changed by adjusting the surface roughness of the rolling mill rolls in the final pass of the cold rolling and applying dry rolling. Regarding No. 2, the rolling temperature was set to 300° C., and the microroughness was further changed. Test pieces of 280 mm×30 mm were collected from the obtained product sheet, and direct-current magnetic measurement was performed by Epstein testing to measure hysteresis loss Wh.sub.10/400 with Bm=1.0 T and f=400 Hz. Moreover, after removing the insulating coating of the product sheet by boiled alkali, surface shape measurement for 100 μm×100 μm was conducted with an accelerating voltage of 5 kV using 3D-SEM (ERA-8800FE) made by Elionix Inc., and the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm was measured under the aforementioned condition. The arithmetic mean roughness Ra of the roll surface in the final pass of the cold rolling was measured by the same method. Further, the arithmetic mean roughness Ra of the steel substrate surface was measured at a scan rate of 0.5 mm/s and a cutoff wavelength of 0.8 mm using a stylus-type roughness meter of 2 μm in stylus tip radius (made by Tokyo Seimitsu Co., Ltd.).

[0090] The results are shown in Table 1. The results indicate that hysteresis loss was low in the disclosed range. In particular, even in the case where Ra of the steel substrate surface measured by the conventional typical measurement technique with cutoff wavelength λc=0.8 mm was 0.2 μm or less, hysteresis loss was high when Ra at cutoff wavelength λc=20 μm defined in the disclosure exceeded 0.2 μm.

TABLE-US-00001 TABLE 1 Ra of steel Ra of steel substrate substrate Ra of surface surface Chemical composition (mass %) roll (μm) (μm) Other surface λc = λc = Wh.sub.10/400 No. C Si Al Mn P S N O components (μm) 20 μm 0.8 mm (W/kg) Remarks 1 0.0017 3.19 0.31 0.54 0.02 0.0023 0.0021 0.0034 0.34 0.36 0.41 6.682 Comparative Example 2 0.0018 3.32 0.14 0.36 0.01 0.0025 0.0019 0.0023 0.13 0.25 0.16 6.562 Comparative Example 3 0.0025 3.24 0.36 0.32 0.01 0.0026 0.0023 0.0031 0.07 0.08 0.12 5.216 Example 4 0.0034 3.45 0.51 0.62 0.02 0.0033 0.0018 0.0016 Sn: 0.08 0.04 0.06 0.15 5.068 Example 5 0.0019 3.32 0.42 0.23 0.01 0.0019 0.0022 0.0024 Sb: 0.06 0.05 0.06 0.13 5.126 Example 6 0.0014 3.18 0.28 0.56 0.06 0.0018 0.0017 0.0019 Ca: 0.0042 0.07 0.09 0.18 5.168 Example 7 0.0023 3.42 0.33 0.42 0.02 0.0024 0.0021 0.0022 Mg: 0.0012 0.10 0.09 0.11 5.098 Example 8 0.0021 3.37 0.44 0.38 0.03 0.0022 0.0016 0.0019 REM: 0.0038 0.06 0.06 0.13 5.142 Example 9 0.0021 3.67 0.25 0.31 0.04 0.0026 0.0014 0.0017 Sn: 0.06 0.08 0.09 0.04 5.042 Example Ca: 0.0031 10 0.0036 3.26 0.21 0.18 0.01 0.0015 0.0031 0.0012 Cr: 6 0.06 0.07 0.22 5.246 Example 11 0.0042 3.43 0.68 0.65 0.01 0.0016 0.0018 0.0023 Ti: 0.31 0.07 0.09 0.15 5.426 Example 12 0.0039 3.29 0.41 0.33 0.01 0.0023 0.0021 0.0026 Nb: 0.26 0.11 0.12 0.16 5.643 Example 13 0.0019 3.59 0.26 0.35 0.02 0.0018 0.0012 0.0034 V: 0.12 0.09 0.11 0.18 5.521 Example Zr: 0.13

Example 3

[0091] A steel slab containing the components shown in Table 2 with the balance consisting of Fe and incidental impurities was obtained by steelmaking, heated at 1100° C. for 30 minutes, and then hot rolled into a hot rolled steel sheet. The hot rolled steel sheet was subjected to hot band annealing of 1000° C.×120 sec, cold rolled to 0.15 mm for No. 1 and to 0.17 mm for Nos. 2 to 12, and then chemically polished to 0.15 mm using a HF+H.sub.2O.sub.2 aqueous solution, thus obtaining a cold rolled steel sheet of 0.15 mm in sheet thickness. The obtained cold rolled steel sheet was subjected to final annealing of 1000° C.×30 sec in an atmosphere of H.sub.2:N.sub.2=30:70 with a dew point of −50° C., and then insulating coating was applied to obtain a product sheet.

[0092] Test pieces of 280 mm×30 mm were collected from the obtained product sheet, and direct-current magnetic measurement was performed by Epstein testing to measure hysteresis loss Wh.sub.10/400 with Bm=1.0 T and f=400 Hz. Moreover, after removing the insulating coating of the product sheet by boiled alkali, surface shape measurement for 100 μm×100 μm was conducted with an accelerating voltage of 5 kV using 3D-SEM (ERA-8800FE) made by Elionix Inc., and the arithmetic mean roughness Ra of the steel substrate surface at cutoff wavelength λc=20 μm was measured under the aforementioned condition. Further, the arithmetic mean roughness Ra of the steel substrate surface was measured at a scan rate of 0.5 mm/s and a cutoff wavelength of 0.8 mm using a stylus-type roughness meter of 2 μm in stylus tip radius (made by Tokyo Seimitsu Co., Ltd.).

[0093] The results are shown in Table 2. In the case of performing chemical polishing, Ra of the steel substrate surface measured by the conventional typical measurement technique with cutoff wavelength λc=0.8 mm was 0.2 μm or more, but hysteresis loss was low when Ra at cutoff wavelength λc=20 μm defined in the disclosure was 0.2 μm or less.

TABLE-US-00002 TABLE 2 Ra of steel Ra of steel substrate substrate surface surface Chemical composition (mass %) (μm) (μm) Other Chemical λc = λc = Wh.sub.10/400 No. C Si Al Mn P S N O components polishing 20 μm 0.8 mm (W/kg) Remarks 1 0.0015 3.26 0.89 0.32 0.01 0.0012 0.0013 0.0021 Not 0.31 0.36 6.428 Compar- applied ative Example 2 0.0013 3.18 1.03 0.26 0.02 0.0015 0.0017 0.0015 Applied 0.06 0.31 5.126 Example 3 0.0023 3.06 0.93 0.25 0.01 0.0018 0.0012 0.0017 Sn: 0.03 Applied 0.02 0.28 5.043 Example 4 0.0014 2.86 1.32 0.65 0.01 0.0005 0.0009 0.0012 Sb: 0.09 Applied 0.04 0.34 5.026 Example 5 0.0018 3.26 0.75 1.32 0.02 0.0019 0.0011 0.0016 Ca: 0.0021 Applied 0.06 0.26 5.044 Example 6 0.0016 3.16 0.87 0.26 0.01 0.0009 0.0015 0.0034 Mg: 0.0008 Applied 0.05 0.29 5.123 Example 7 0.0013 3.06 0.95 0.76 0.01 0.0015 0.0023 0.0029 REM: 0.0026 Applied 0.09 0.33 5.064 Example 8 0.0014 2.95 0.88 0.46 0.03 0.0016 0.0013 0.0019 Sn: 0.05 Applied 0.07 0.27 5.033 Example Ca: 0.0036 9 0.0019 2.63 1.12 0.26 0.01 0.0014 0.0019 0.0016 Cr: 5.2 Applied 0.03 0.26 5.213 Example 10 0.0026 3.12 0.65 0.89 0.01 0.0019 0.0012 0.0017 Ti: 0.57 Applied 0.08 0.29 5.326 Example 11 0.0022 3.42 0.87 0.42 0.01 0.0011 0.0024 0.0025 Nb: 0.46 Applied 0.11 0.32 5.541 Example 12 0.0014 3.22 0.84 0.72 0.01 0.0015 0.0014 0.0029 V: 0.09 Applied 0.12 0.35 5.426 Example Zr: 0.05

INDUSTRIAL APPLICABILITY

[0094] The disclosed non-oriented electrical steel sheet has iron loss reduced by reducing the microroughness of the steel substrate surface, without significantly limiting the steel components. This advantageous effect is attained by a principle different from increasing specific resistance or reducing sheet thickness. Accordingly, the use of the disclosed technique together with these techniques can further reduce iron loss.