MANUFACTURING METHOD OF GRAIN-ORIENTED ELECTRICAL STEEL SHEET

Abstract

This manufacturing method of a grain-oriented electrical steel sheet is a manufacturing method of a grain-oriented electrical steel sheet having no forsterite film, in which in a decarburization annealing process, a cold-rolled steel sheet is (i-1) heated at an average heating rate (HR1) of 40 to 500° C./sec in a temperature range of 550° C. or higher and lower than 720° C., (i-2) heated at an average heating rate (HR2), which is 5 to 50° C./sec, in a temperature range of 720° C. to T1° C. (770° C.≤T1 (° C.)≤900° C.), and (ii) retained at the temperature of T1° C. for 50 to 1000 seconds to make an amount of C 25 ppm or less in an atmosphere of an oxygen partial pressure (P1) which is 0.0010 to 0.20.

Claims

1. A manufacturing method of a grain-oriented electrical steel sheet comprising: a hot rolling process in which a steel piece containing 0.10% by mass or less of C, 0.80 to 7.00% by mass of Si, 0.01 to 0.07% by mass of acid-soluble Al, 0.012% by mass or less of N, 1.00% by mass or less of Mn, and 0.08% by mass or less of S, and including a remainder of Fe and impurities is hot-rolled into a hot-rolled steel sheet; an annealing process in which the hot-rolled steel sheet is annealed; a pickling process in which the hot-rolled steel sheet after the annealing process is pickled; a cold rolling process in which the hot-rolled steel sheet after the pickling process is cold-rolled into a cold-rolled steel sheet; a decarburization annealing process in which the cold-rolled steel sheet is decarburized; and a final annealing process in which the steel sheet after the decarburization annealing process is final-annealed, wherein the decarburization annealing process includes: (i-1) a first heat treatment of heating at an average heating rate HR1, which is 40 to 500° C./sec, in a temperature range of 550° C. or higher and lower than 720° C.; (i-2) a second heat treatment of heating at an average heating rate HR2, which is 5 to 50° C./sec, in a temperature range of 720° C. or higher and a temperature T1° C. or lower that satisfies the following expression (2) following the first heat treatment; and (ii) a first annealing treatment of retaining the temperature at T1° C. for 50 to 1000 seconds following the second heat treatment, the first heat treatment, the second heat treatment, and the first annealing treatment are performed in an atmosphere of an oxygen partial pressure P1 satisfying the following expression (1), and an amount of C in the steel sheet after the first annealing treatment is 25 ppm or less
0.0010≤P1≤0.20   (1),
770≤T1 (° C.)≤900   (2).

2. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1, wherein the decarburization annealing process further includes a second annealing treatment of retaining a temperature T2° C. satisfying the following expression (4) for 3 to 500 seconds in an atmosphere of an oxygen partial pressure P2 satisfying the following expression (3) after the first annealing treatment,
P2<P1   (3),
960≥T2≥T1+10   (4).

3. The manufacturing method of a grain-oriented electrical steel sheet according to claim 2, further comprising a third heat treatment between the first annealing treatment and the second annealing treatment, wherein the third heat treatment performs heating at an average heating rate HR3, which is 5 to 50° C./sec, in a temperature range from T1° C. to T2° C.

4. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1, wherein the steel piece contains one or more of 0.01 to 0.50% by mass of Cr, 0.01 to 0.50% by mass of Cu, and 0.01 to 0.02% by mass of Sn.

5. The manufacturing method of a grain-oriented electrical steel sheet according to claim 1, further comprising a nitriding treatment process of nitriding the steel sheet after the decarburization annealing process between the decarburization annealing process and the final annealing process, wherein the steel sheet after the nitriding treatment process is final-annealed in the final annealing process.

Description

EXAMPLE

[0116] Hereinafter, technical contents of the present manufacturing method will be further described on the basis of examples of the present manufacturing method.

[0117] Further, conditions in the examples described below are condition examples employed for ascertaining feasibility and effects of the present invention, and the present invention is not limited to the condition examples. Also, the present invention can employ various conditions as long as the objective of the present invention is achieved without departing from the gist of the present invention.

Example 1

[0118] An ingot having the chemical composition shown in Table 1 was vacuum-melted and casted into a steel piece. The steel piece was heated to 1150° C. and subjected to hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of 2.3 mm. The hot-rolled steel sheet was annealed at 1000 to 1100° C. for 2 minutes, pickled, and then subjected to cold rolling to obtain a cold-rolled steel sheet having a final sheet thickness of 0.23 to 0.30 mm.

TABLE-US-00001 TABLE 1 Steel No. C Si Mn Al S N Others Invention A1 0.005 3.15 0.08 0.028 0.021 0.007 steel A2 0.095 3.16 0.08 0.027 0.018 0.008 A3 0.065 1.01 0.09 0.027 0.018 0.007 A4 0.078 6.38 0.11 0.028 0.015 0.009 A5 0.037 2.67 0.03 0.029 0.017 0.008 A6 0.034 2.99 0.95 0.029 0.018 0.007 A7 0.035 3.14 0.06 0.012 0.018 0.008 A8 0.058 3.18 0.45 0.068 0.019 0.009 A9 0.045 3.12 0.09 0.033 0.003 0.008 A10 0.055 3.16 0.07 0.039 0.071 0.006 A11 0.057 3.26 0.08 0.027 0.039 0.002 A12 0.037 3.07 0.12 0.028 0.037 0.011 A13 0.036 3.17 0.09 0.028 0.018 0.010 A14 0.029 3.11 0.07 0.029 0.017 0.004 A15 0.051 3.15 0.09 0.028 0.015 0.007 Cr: 0.05 A16 0.055 3.22 0.17 0.029 0.012 0.006 Cr: 0.14, Cu: 0.08 A17 0.059 3.45 0.08 0.029 0.014 0.007 Cu: 0.32, Sn: 0.03 Compar- a1 0.135 3.05 0.09 0.029 0.018 0.009 ative a2 0.032 0.77 0.08 0.029 0.015 0.008 steel a3 0.048 7.12 0.12 0.027 0.019 0.007 a4 0.037 3.17 1.21 0.028 0.017 0.006 a5 0.045 3.11 0.08 0.008 0.015 0.008 a6 0.042 3.07 0.09 0.087 0.014 0.007 a7 0.056 3.17 0.07 0.029 0.091 0.009 a8 0.034 3.26 0.12 0.028 0.018 0.015

[0119] The cold-rolled steel sheet was subjected to decarburization annealing at 820° C. for 140 seconds in a wet gas containing hydrogen and nitrogen (P1=P.sub.H2O/P.sub.H2=0.10). The average heating rate HR1 in the temperature range of 550° C. or higher and lower than 720° C. was set to 80° C./sec, and the average heating rate HR2 in the temperature range of 720° C. or higher and 820° C. or lower (that is, T1: 820° C.) was set to 10° C./sec.

[0120] An annealing separator containing alumina as a main component was applied to the steel sheet after the decarburization annealing in a form of an aqueous slurry, and final annealing was performed. The final annealing was performed up to 1200° C. at a temperature rising rate of 15° C./hour in an atmosphere containing nitrogen, and the atmosphere was switched to hydrogen 100% at 1200° C. to perform annealing for 20 hours.

[0121] A sample for evaluation was taken from the secondary recrystallized steel sheet that had undergone the above processes, the sample was subjected to strain relief annealing at 800° C. for 2 hours in a dry gas atmosphere containing nitrogen and hydrogen, an aging treatment was performed at 150° C. for 300 hours in a nitrogen atmosphere thereafter, and then a magnetic flux density and iron loss were measured using a single sheet tester (SST), and furthermore a residual amount of carbon was analyzed.

[0122] Table 2 shows magnetic flux densities B8(T), iron loss (W.sub.17/50), and residual amounts of carbon (ppm) after the aging treatment.

TABLE-US-00002 TABLE 2 Residual Magnetic amount of flux Iron loss Sheet Steel carbon [C] density W.sub.17/50 thickness No. No. (ppm) B8 (T) (W/Kg) (mm) Invention B1 A1 22 1.89 0.82 0.30 example B2 A2 21 1.91 0.81 0.30 B3 A3 15 1.95 0.84 0.30 B4 A4 18 1.89 0.77 0.30 B5 A5 22 1.96 0.82 0.30 B6 A6 23 1.90 0.79 0.30 B7 A7 15 1.89 0.81 0.30 B8 A8 18 1.89 0.80 0.30 B9 A9 22 1.90 0.80 0.30 B10 A10 21 1.91 0.81 0.30 B11 A11 20 1.94 0.82 0.30 B12 A12 22 1.93 0.81 0.30 B13 A13 12 1.93 0.80 0.30 B14 A14  9 1.92 0.82 0.30 B15 A15 14 1.93 0.79 0.30 B16 A16 14 1.94 0.75 0.30 B17 A17 14 1.93 0.75 0.30 B18 A17 17 1.95 0.74 0.27 B19 A17 18 1.94 0.73 0.27 B20 A17 19 1.94 0.73 0.23 B21 A17 21 1.96 0.72 0.23 Compar- b1 a1 47 1.67 — 0.30 ative b2 a2 19 1.55 — 0.30 example b3 a3 — — — 0.30 b4 a4 23 1.49 — 0.30 b5 a5 24 1.51 — 0.30 b6 a6 — — — 0.30 b7 a7 — — — 0.30 b8 a8 — — — 0.30

[0123] The analysis of the residual amount of carbon was performed according to JIS G 1211-4: 2011. From the perspective of magnetic aging, a target residual amount of carbon was 25 ppm or less. The SST was performed according to JIS C 2553. The iron loss was evaluated by the iron loss W.sub.17/50 (W/kg) at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T. A target for W.sub.17/50 (W/kg) was 0.85 or less.

[0124] The magnetic flux density was evaluated by the magnetic flux density B8 at a magnetic field intensity of 800 A/m. The B8 was aimed at 1.88 T or higher, and the iron loss was not evaluated for a sample with the B8 less than 1.88 T. Also, for a steel sheet on which the cold rolling could not be performed, the residual amount of carbon was not analyzed, and the magnetic flux density and the iron loss were not evaluated. In the invention example, it is ascertained that the residual amount of carbon of 23 ppm or less, the magnetic flux density B8 of 1.89 T or more, and the iron loss W.sub.17/50 (W/kg) of 0.84 or less have been obtained.

Example 2

[0125] An ingot having the chemical composition shown in Table 1 was vacuum-melted and casted into a steel piece. The steel piece was heated to 1150° C. and subjected to hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of 2.3 mm. The hot-rolled steel sheet was annealed at 1000 to 1100° C. for 2 minutes, pickled, and then subjected to cold rolling to obtain a cold-rolled steel sheet having a final sheet thickness of 0.23 to 0.30 mm.

[0126] This cold-rolled steel sheet was subjected to decarburization annealing under the conditions shown in Table 3. HR1 is an average heating rate in a temperature range of 550° C. or higher and lower than 720° C., and HR2 is an average heating rate in a temperature range of 720° C. or higher and an annealing temperature T1° C. or lower. Also, the “retention time” in Table 3 represents a retention time at T1° C. during the decarburization annealing.

TABLE-US-00003 TABLE 3 Decarburization anneal Oxygen Residual Magnetic partial Retention Retention amount of flux Iron loss Sheet Steel pressure temperature time HR1 HR2 carbon [C] density W.sub.17/50 thickness No. No. P1 T1 (° C.) (sec) (° C./sec) (° C./sec) (ppm) B8 (T) (W/Kg) (mm) Invention C1 A15 0.002 790 150 54 12 22 1.90 0.76 0.30 example C2 A15 0.186 788 140 55 10 18 1.91 0.77 0.30 C3 A15 0.005 775 150 60 9 19 1.91 0.78 0.30 C4 A15 0.008 880 140 56 9 18 1.92 0.78 0.30 C5 A15 0.05 803 55 68 11 20 1.92 0.78 0.30 C6 A15 0.075 806 150 55 12 19 1.90 0.76 0.30 C7 A15 0.065 800 120 43 14 20 1.91 0.76 0.30 C8 A15 0.08 807 120 120 6 13 1.93 0.75 0.30 C9 A17 0.12 825 125 155 12 12 1.94 0.73 0.30 C10 A17 0.11 820 120 150 13 17 1.95 0.71 0.27 C11 A17 0.12 830 125 155 11 19 1.96 0.69 0.23 C12 A15 0.11 825 995 65 10 18 1.92 0.77 0.30 C13 A15 0.12 890 120 495 12 17 1.91 0.78 0.30 C14 A15 0.12 775 120 60 45 18 1.92 0.77 0.30 Compar- c1 A15 0.0008 786 142 60 12 89 1.90 0.88 0.30 ative c2 A15 0.235 795 143 59 15 77 1.91 0.89 0.30 example c3 A15 0.007 750 145 55 12 86 1.90 0.91 0.30 c4 A15 0.008 920 147 47 15 77 1.89 0.91 0.30 c5 A15 0.007 865 40 45 12 67 1.89 0.88 0.30 c6 A15 0.007 860 1100 43 10 17 1.56 — 0.30 c7 A15 0.009 871 155 30 20 89 1.90 0.87 0.30 c8 A15 0.005 859 145 55 3 68 1.92 0.95 0.30 c9 A15 0.007 910 120 600 — 73 1.87 — 0.30 c10 A15 0.007 915 120 60 80 69 1.86 — 0.30

[0127] An annealing separator containing alumina as a main component was applied to the steel sheet after the decarburization annealing in a form of an aqueous slurry, and final annealing was performed. The final annealing was performed up to 1200° C. at a temperature rising rate of 15° C./hour in an atmosphere containing nitrogen, and the atmosphere was switched to hydrogen 100% at 1200° C. to perform annealing for 20 hours.

[0128] A sample of the evaluation table was taken from the secondary recrystallized steel sheet that had undergone the above processes, the sample was subjected to strain relief annealing at 800° C. for 2 hours in a dry gas atmosphere containing nitrogen and hydrogen, an aging treatment was performed at 150° C. for 300 hours in a nitrogen atmosphere thereafter, and then a magnetic flux density and iron loss were measured using the SST, and furthermore a residual amount of carbon was analyzed. A method of evaluating magnetism and a method of analyzing the residual amount of carbon are the same as those in Example 1.

[0129] Table 3 shows magnetic flux densities B8(T), iron loss (W.sub.17/50), and residual amounts of carbon (ppm) after the aging treatment in combination. In the invention example, it is ascertained that the residual amount of carbon of 22 ppm or less, the magnetic flux density B8 of 1.90 T or more, and the iron loss W.sub.17/50 (W/kg) of 0.78 or less have been obtained. Further, in comparative example c9, the average heating rate HR1 exceeded 500° C./sec, and a temperature of the steel sheet exceeded the upper limit value of T1 when it is heated at the average heating rate HR1. Therefore, the average heating rate HR2 could not be measured.

Example 3

[0130] An ingot having the chemical composition shown in Table 1 was vacuum-melted and casted into a steel piece. The steel piece was heated to 1150° C. and subjected to hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of 2.3 mm. The hot-rolled steel sheet was annealed at 1000 to 1100° C. for 2 minutes, pickled, and then subjected to cold rolling to obtain a cold-rolled steel sheet having a final sheet thickness of 0.23 to 0.30 mm.

[0131] This cold-rolled steel sheet was subjected to decarburization annealing at T1=820° C. for 140 seconds in an annealing atmosphere of P1=0.10. At that time, heating up to T1 was performed at the average heating rate HR1 of 80° C./sec in the temperature range of 550° C. or higher and lower than 720° C. and was performed at the average heating rate HR2 of 10° C./sec in the temperature range of 720° C. or higher and 820° C. or lower.

[0132] The cold-rolled steel sheet was subjected to decarburization annealing at 820° C. for 140 seconds, and then subsequently subjected to a second annealing treatment under the conditions shown in Table 4 without lowering the retention temperature.

TABLE-US-00004 TABLE 4 Second annealing treatment Oxygen Residual Magnetic partial Retention amount of flux Iron loss Sheet Steel HR3 pressure temperature Retention carbon [C] density W.sub.17/50 thickness No. No. (° C./sec) P2 T2 (° C.) time (sec) (ppm) B8 (T) (W/Kg) (mm) Invention D1 A15 6 0.056 845 150 5 1.90 0.74 0.30 example D2 A15 45 0.035 850 140 3 1.91 0.73 0.30 D3 A15 6 0.081 856 5 4 1.93 0.72 0.30 D4 A15 7 0.041 835 150 2 1.91 0.74 0.30 D5 A15 7 0.038 860 5 4 1.92 0.73 0.30 D6 A15 8 0.037 870 190 3 1.90 0.72 0.30 D7 A17 17 0.002 865 15 2 1.94 0.70 0.30 D8 A15 3 0.001 875 15 10 1.90 0.76 0.30 D9 A15 4 0.002 871 86 12 1.91 0.76 0.30 D10 A15 4 0.115 889 156 22 1.90 0.79 0.30 D11 A15 4 0.034 820 145 24 1.90 0.80 0.30 D12 A15 65 0.035 960 147 22 1.89 0.79 0.30 D13 A15 71 0.041 894 1 23 1.89 0.79 0.30 D14 A17 16 0.002 860 17 10 1.94 0.68 0.27 D15 A17 18 0.002 870 16 18 1.97 0.65 0.23 D16 A17 8 0.002 870 495 3 1.93 0.72 0.30

[0133] An annealing separator containing alumina as a main component was applied to the steel sheet after the second annealing treatment in a fonn of an aqueous slurry, and final annealing was performed. The final annealing was performed up to 1200° C. while raising the temperature at a temperature rising rate of 15° C./hour in an atmosphere containing nitrogen, and the atmosphere was switched to hydrogen 100% at 1200° C. to perform annealing for 20 hours.

[0134] A sample for evaluation was taken from the secondary recrystallized steel sheet that had undergone the above processes, the sample was subjected to strain relief annealing at 800° C. for 2 hours in a dry gas atmosphere containing nitrogen and hydrogen, an aging treatment was performed at 150° C. for 300 hours in a nitrogen atmosphere thereafter, and then a magnetic flux density and iron loss were measured using the SST, and furthermore a residual amount of carbon was analyzed. A method of evaluating magnetism and a method of analyzing the residual carbon are the same as those in example 1.

[0135] Table 4 shows magnetic flux densities B8(T), iron loss (W.sub.17/50), and residual amounts of carbon (ppm) of the steel sheets after the aging treatment in combination. It is ascertained that the residual amount of carbon of 24 ppm or less, the magnetic flux density B8 of 1.89 T or more, and the iron loss W.sub.17/50 (W/kg) of 0.80 or less have been obtained. Particularly, in invention examples D1 to D7 and D14 to D16 that satisfy preferable conditions of the second annealing treatment and the third heat treatment described in the present embodiment, the residual amount of carbon or the iron loss tended to be further reduced. Further, sheet thicknesses decreased in D14 and D15, and in this respect also, iron loss became satisfactory values.

Example 4

[0136] In Example 4, the same treatment as in invention examples D1 to D16 of example 3 was performed except that a nitriding treatment was performed between the second annealing treatment and the final annealing process. Here, the nitriding treatment was performed by retaining the steel sheet at 700 to 800° C. for 30 seconds in an ammonia gas atmosphere. The results are shown in Table 5. In any of the invention examples, the residual amount of carbon was 25 ppm or less, the magnetic flux density B8 was 1.88 T or more, and the iron loss W.sub.17/50 (W/kg) was 0.85 or less.

TABLE-US-00005 TABLE 5 Residual Magnetic amount of flux Iron loss Sheet Steel carbon [C] density W.sub.17/50 thickness No. No. (ppm) B8 (T) (W/Kg) (mm) Invention E1 A15 6 1.91 0.75 0.30 example E2 A15 4 1.92 0.74 0.30 E3 A15 5 1.92 0.73 0.30 E4 A15 1 1.91 0.74 0.30 E5 A15 3 1.91 0.74 0.30 E6 A15 4 1.91 0.71 0.30 E7 A17 3 1.93 0.72 0.30 E8 A15 11 1.90 0.75 0.30 E9 A15 13 1.91 0.76 0.30 E10 A15 21 1.91 0.78 0.30 E11 A15 23 1.90 0.79 0.30 E12 A15 21 1.89 0.80 0.30 E13 A15 24 1.90 0.79 0.30 E14 A17 11 1.93 0.69 0.27 E15 A17 17 1.96 0.65 0.23 E16 A17 4 1.92 0.72 0.30

INDUSTRIAL APPLICABILITY

[0137] As described above, according to the present invention, in the manufacturing method of a grain-oriented electrical steel sheet having no forsterite film by intentionally preventing generation of Fe-based oxides on a surface of the steel sheet by annealing in an atmosphere of a low oxygen partial pressure in the decarburization annealing process after the cold rolling, the decarburization annealing can be stably performed without going through two or more instances of cold rolling including an intermediate annealing even in an atmosphere of a low oxygen partial pressure in which generation of Fe-based oxides is suppressed. Therefore, the present invention has high applicability in the electrical steel sheet manufacturing industry.