METHOD OF PRODUCING GRAIN-ORIENTED ELECTRICAL STEEL SHEET

Abstract

A grain-oriented electrical steel sheet has magnetic properties improved over conventional grain-oriented electrical steel sheets. A method of producing a grain-oriented electrical steel sheet comprises: heating a steel slab at 1300? C. or less, the steel slab having a chemical composition containing C, Si, Mn, acid-soluble Al, S and/or Se, Sn and/or Sb, N, and a balance being Fe and inevitable impurities; subjecting the steel slab to hot rolling to obtain a hot rolled steel sheet; subjecting the hot rolled steel sheet to cold rolling once, or twice or more with intermediate annealing performed therebetween, to obtain a cold rolled steel sheet with a final sheet thickness; subjecting the cold rolled steel sheet to primary recrystallization annealing; applying an annealing separator to a surface of the cold rolled steel sheet after the primary recrystallization annealing; and then subjecting the cold rolled steel sheet to secondary recrystallization annealing.

Claims

1.-8. (canceled)

9. A method of producing a grain-oriented electrical steel sheet, the method comprising: heating a steel slab at 1300? C. or less, the steel slab having a chemical composition containing, in mass %, C in an amount of 0.002% or more and 0.080% or less, Si in an amount of 2.00% or more and 8.00% or less, Mn in an amount of 0.02% or more and 0.50% or less, acid-soluble Al in an amount of 0.003% or more and less than 0.010%, S and/or Se in an amount of 0.005% or more and 0.010% or less in total, Sn and/or Sb in an amount of 0.005% or more and 1.000% or less in total, N in an amount of less than 0.006%, and a balance being Fe and inevitable impurities; subjecting the steel slab to hot rolling to obtain a hot rolled steel sheet; subjecting the hot rolled steel sheet to cold rolling once, or twice or more with intermediate annealing performed therebetween, to obtain a cold rolled steel sheet with a final sheet thickness; subjecting the cold rolled steel sheet to primary recrystallization annealing; applying an annealing separator to a surface of the cold rolled steel sheet after subjection to the primary recrystallization annealing; and then subjecting the cold rolled steel sheet to secondary recrystallization annealing.

10. The method of producing a grain-oriented electrical steel sheet according to claim 9, wherein in the chemical composition, the total amount of Sn and/or Sb is in a range of 0.020% or more and 0.300% or less in mass %.

11. The method of producing a grain-oriented electrical steel sheet according to claim 9, wherein the chemical composition further contains, in mass %, one or more selected from Ni in an amount of 0.005% or more and 1.5% or less, Cu in an amount of 0.005% or more and 1.5% or less, Cr in an amount of 0.005% or more and 0.1% or less, P in an amount of 0.005% or more and 0.5% or less, Mo in an amount of 0.005% or more and 0.5% or less, Ti in an amount of 0.0005% or more and 0.1% or less, Nb in an amount of 0.0005% or more and 0.1% or less, V in an amount of 0.0005% or more and 0.1% or less, B in an amount of 0.0002% or more and 0.0025% or less, Bi in an amount of 0.005% or more and 0.1% or less, Te in an amount of 0.0005% or more and 0.01% or less, and Ta in an amount of 0.0005% or more and 0.01% or less.

12. The method of producing a grain-oriented electrical steel sheet according to claim 10, wherein the chemical composition further contains, in mass %, one or more selected from Ni in an amount of 0.005% or more and 1.5% or less, Cu in an amount of 0.005% or more and 1.5% or less, Cr in an amount of 0.005% or more and 0.1% or less, P in an amount of 0.005% or more and 0.5% or less, Mo in an amount of 0.005% or more and 0.5% or less, Ti in an amount of 0.0005% or more and 0.1% or less, Nb in an amount of 0.0005% or more and 0.1% or less, V in an amount of 0.0005% or more and 0.1% or less, B in an amount of 0.0002% or more and 0.0025% or less, Bi in an amount of 0.005% or more and 0.1% or less, Te in an amount of 0.0005% or more and 0.01% or less, and Ta in an amount of 0.0005% or more and 0.01% or less.

13. The method of producing a grain-oriented electrical steel sheet according to claim 9, further comprising after the cold rolling, subjecting the cold rolled steel sheet to nitriding treatment.

14. The method of producing a grain-oriented electrical steel sheet according to claim 10, further comprising after the cold rolling, subjecting the cold rolled steel sheet to nitriding treatment.

15. The method of producing a grain-oriented electrical steel sheet according to claim 11, further comprising after the cold rolling, subjecting the cold rolled steel sheet to nitriding treatment.

16. The method of producing a grain-oriented electrical steel sheet according to claim 12, further comprising after the cold rolling, subjecting the cold rolled steel sheet to nitriding treatment.

17. The method of producing a grain-oriented electrical steel sheet according to claim 9, wherein one or more selected from sulfide, sulfate, selenide, and selenate are added to the annealing separator.

18. The method of producing a grain-oriented electrical steel sheet according to claim 10, wherein one or more selected from sulfide, sulfate, selenide, and selenate are added to the annealing separator.

19. The method of producing a grain-oriented electrical steel sheet according to claim 11, wherein one or more selected from sulfide, sulfate, selenide, and selenate are added to the annealing separator.

20. The method of producing a grain-oriented electrical steel sheet according to claim 12, wherein one or more selected from sulfide, sulfate, selenide, and selenate are added to the annealing separator.

21. The method of producing a grain-oriented electrical steel sheet according to claim 9, further comprising after the cold rolling, subjecting the cold rolled steel sheet to magnetic domain refining treatment.

22. The method of producing a grain-oriented electrical steel sheet according to claim 10, further comprising after the cold rolling, subjecting the cold rolled steel sheet to magnetic domain refining treatment.

23. The method of producing a grain-oriented electrical steel sheet according to claim 11, further comprising after the cold rolling, subjecting the cold rolled steel sheet to magnetic domain refining treatment.

24. The method of producing a grain-oriented electrical steel sheet according to claim 12, further comprising after the cold rolling, subjecting the cold rolled steel sheet to magnetic domain refining treatment.

25. The method of producing a grain-oriented electrical steel sheet according to claim 23, wherein in the magnetic domain refining treatment, the cold rolled steel sheet after subjection to the secondary recrystallization annealing is irradiated with an electron beam.

26. The method of producing a grain-oriented electrical steel sheet according to claim 24, wherein in the magnetic domain refining treatment, the cold rolled steel sheet after subjection to the secondary recrystallization annealing is irradiated with an electron beam.

27. The method of producing a grain-oriented electrical steel sheet according to claim 23, wherein in the magnetic domain refining treatment, the cold rolled steel sheet after subjection to the secondary recrystallization annealing is irradiated with a laser.

28. The method of producing a grain-oriented electrical steel sheet according to claim 24, wherein in the magnetic domain refining treatment, the cold rolled steel sheet after subjection to the secondary recrystallization annealing is irradiated with a laser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the accompanying drawings:

[0033] FIG. 1 is a graph illustrating the influence of the amount of Sn+Sb in a raw material on the magnetic flux density B.sub.8 of a product sheet.

DETAILED DESCRIPTION

[0034] [Chemical Composition]

[0035] A method of producing a grain-oriented electrical steel sheet according to one of the disclosed embodiments is described below. The reasons for limiting the chemical composition of steel are described first. In the description, % representing the content (amount) of each component element denotes mass % unless otherwise noted.

[0036] C in an amount of 0.002% or more and 0.080% or less

[0037] If the amount of C is less than 0.002%, the grain boundary strengthening effect by C is lost, and defects which hamper production, such as slab cracking, appear. If the amount of C is more than 0.080%, it is difficult to reduce, by decarburization annealing, the amount to 0.005% or less that causes no magnetic aging. The amount of C is therefore preferably in a range of 0.002% or more and 0.080% or less.

[0038] Si in an amount of 2.00% or more and 8.00% or less

[0039] Si is a very effective element in increasing the electrical resistance of the steel and reducing eddy current loss which constitutes part of iron loss. When adding Si to the steel sheet, the electrical resistance monotonically increases until the amount of Si reaches 11%. Once the amount of Si exceeds 8.00%, however, workability decreases significantly. If the amount of Si is less than 2.00%, the electrical resistance is low, and good iron loss properties cannot be obtained. The amount of Si is therefore in a range of 2.00% or more and 8.00% or less. The amount of Si is more preferably in a range of 2.50% or more and 4.50% or less.

[0040] Mn in an amount of 0.02% or more and 0.50% or less

[0041] Mn bonds with S or Se to form MnS or MnSe. Such MnS or MnSe, even in a minute amount, acts to inhibit normal grain growth in the heating process of secondary recrystallization annealing, in combination use with a grain boundary segregation element. If the amount of Mn is less than 0.02%, the normal grain growth inhibiting capability is insufficient. If the amount of Mn is more than 0.50%, not only high-temperature slab heating is necessary in the slab heating process before hot rolling in order to completely dissolve Mn, but also MnS or MnSe forms as a coarse precipitate, and thus the normal grain growth inhibiting capability decreases. The amount of Mn is therefore in a range of 0.02% or more and 0.50% or less.

[0042] S and/or Se in an amount of 0.005% or more and 0.010% or less in total

[0043] S and Se are one of the features of the present disclosure. As mentioned above, S and Se bond with Mn to exert the normal grain growth inhibiting action. If the total amount of S and/or Se is less than 0.005%, the normal grain growth inhibiting capability is insufficient. The total amount of S and/or Se is therefore preferably 0.005% or more. If the total amount of S and/or Se is more than 0.010%, MnS or MnSe cannot dissolve completely in the low-temperature slab heating process at 1300? C. or less which is one of the features of the present disclosure, causing insufficient normal grain growth inhibiting capability. The total amount of S and/or Se is therefore in a range of 0.005% or more and 0.010% or less.

[0044] sol.Al in an amount of 0.003% or more and less than 0.010%

[0045] Al forms a dense oxide film on the surface, and can make the control of nitriding content difficult during nitriding or hamper decarburization. Accordingly, the amount of Al is limited to less than 0.010% in sol.Al amount. Al having high oxygen affinity is, when added in a minute amount in steelmaking, expected to reduce the amount of dissolved oxygen in the steel and, for example, reduce oxide inclusions which cause degradation in properties. In view of this, the amount of sol.Al is 0.003% or more, with it being possible to suppress degradation in magnetic properties.

[0046] N in an amount of less than 0.006%

[0047] If the amount of N is excessively high, secondary recrystallization becomes difficult, as with S and Se. In particular, if the amount of N is 0.006% or more, secondary recrystallization is unlikely to occur, and the magnetic properties degrade. The amount of N is therefore limited to less than 0.006%.

[0048] At least one of Sn and Sb: Sn and/or Sb in an amount of 0.005% or more and 1.000% or less in total

[0049] Sn and Sb are one of the features of the present disclosure. Sn and Sb are grain boundary segregation elements. Adding these elements increases the normal grain growth inhibiting capability and enhances the secondary recrystallization driving force, thus stabilizing secondary recrystallization. If the total amount of Sn and/or Sb is less than 0.005%, the effect of the normal grain growth inhibiting capability is insufficient. If the total amount of Sn and/or Sb is more than 1.000%, excessive normal grain growth inhibiting capability causes unstable secondary recrystallization, leading to degradation in magnetic properties. Besides, productivity drops due to grain boundary embrittlement or rolling load increase. The total amount of Sn and/or Sb is therefore in a range of 0.005% or more and 1.000% or less. The total amount of Sn and/or Sb is more preferably in a range of 0.020% or more and 0.300% or less, in terms of magnetic property scattering reduction and productivity.

[0050] An experiment that led to limiting the amount of Sn and Sb to the above-mentioned range is described below.

[0051] Table 1 illustrates the magnetic flux density B.sub.8 of a product sheet that varies depending on the amount of Sn+Sb. A slab with a thickness of 220 mm of each steel listed in Table 1 with the balance being Fe and inevitable impurities was heated to 1200? C., and then hot rolled to a thickness of 2.5 mm. After this, the hot rolled sheet was hot band annealed at 1000? C. for 60 s, and then cold rolled to a thickness of 0.27 mm. The cold rolled sheet was then subjected to primary recrystallization annealing at 820? C. for 100 s. The heating rate from 500? C. to 700? C. in the primary recrystallization annealing was 200? C./s. Subsequently, an annealing separator mainly composed of MgO was applied to the steel sheet surface, and then the steel sheet was subjected to secondary recrystallization annealing serving also as purification annealing at 1200? C. for 10 h. Following this, a phosphate-based insulating tension coating was applied and baked on the steel sheet, and flattening annealing was performed for the purpose of flattening the steel strip to obtain a product. Test pieces were thus obtained under the respective conditions.

TABLE-US-00001 TABLE 1 Secondary recrystallization Chemical composition (mass %) annealed sheet sol. B.sub.8 W.sub.17/50 No. Si C Mn Al N S Se Sn Sb S + Se Sn + Sb (T) (W/kg) Remarks 1 3.41 0.045 0.07 0.007 0.003 0.006 0 0.002 0.002 0.006 0.004 1.857 0.987 Comparative Example 2 3.38 0.041 0.08 0.008 0.004 0.001 0.007 0.000 0.015 0.008 0.015 1.891 0.902 Example 3 3.43 0.043 0.08 0.008 0.004 0.009 0.001 0.003 0.025 0.010 0.028 1.903 0.888 Example 4 3.36 0.046 0.09 0.008 0.004 0.002 0.001 0.002 0.033 0.003 0.035 1.867 0.938 Comparative Example 5 3.40 0.052 0.07 0.007 0.005 0.005 0.003 0.036 0.002 0.008 0.038 1.904 0.883 Example 6 3.41 0.042 0.08 0.007 0.004 0.005 0.003 0.002 0.048 0.008 0.050 1.911 0.872 Example 7 3.44 0.049 0.08 0.009 0.004 0.002 0.001 0.002 0.051 0.003 0.053 1.869 0.936 Comparative Example 8 3.39 0.034 0.08 0.007 0.005 0.006 0.001 0.003 0.077 0.007 0.080 1.917 0.859 Example 9 3.43 0.044 0.09 0.008 0.004 0.011 0.001 0.002 0.079 0.012 0.081 1.544 2.439 Comparative Example 10 3.38 0.041 0.08 0.006 0.004 0.002 0.004 0.062 0.077 0.006 0.139 1.919 0.851 Example 11 3.37 0.052 0.09 0.009 0.004 0.003 0.001 0.150 0.076 0.004 0.226 1.859 0.966 Comparative Example 12 3.42 0.055 0.09 0.009 0.005 0.006 0 0.150 0.083 0.006 0.233 1.917 0.855 Example 13 3.36 0.048 0.08 0.008 0.004 0.002 0.010 0.160 0.079 0.012 0.239 1.706 1.824 Comparative Example 14 3.40 0.033 0.08 0.008 0.003 0.005 0.003 0.350 0.110 0.008 0.460 1.896 0.916 Example 15 3.38 0.035 0.07 0.008 0.004 0.002 0.002 0.370 0.200 0.004 0.570 1.853 0.960 Comparative Example 16 3.29 0.044 0.08 0.003 0.004 0.006 0.002 0.005 0.005 0.008 0.010 1.884 0.918 Example 17 3.44 0.049 0.08 0.004 0.005 0.006 0.001 0.001 0.024 0.007 0.025 1.911 0.870 Example 18 3.41 0.049 0.07 0.008 0.004 0.004 0.001 0.500 0.100 0.005 0.600 1.892 0.945 Example 19 3.40 0.050 0.08 0.007 0.004 0.002 0.007 0.250 0.050 0.009 0.300 1.906 0.889 Example 20 3.33 0.042 0.09 0.009 0.004 0.002 0.006 0.003 0.002 0.008 0.005 1.883 0.935 Example 21 3.36 0.039 0.09 0.009 0.004 0.006 0.001 0.750 0.250 0.007 1.000 1.882 0.931 Example 22 3.35 0.045 0.08 0.008 0.004 0.007 0.002 0.750 0.350 0.009 1.100 1.872 0.975 Comparative Example 23 3.39 0.051 0.08 0.007 0.005 0 0.006 0.011 0 0.006 0.011 1.894 0.944 Example 24 3.45 0.049 0.09 0.009 0.004 0.002 0.005 0 0.013 0.007 0.013 1.882 0.934 Example 25 3.42 0.048 0.09 0.009 0.004 0.001 0.004 0.014 0 0.005 0.014 1.885 0.917 Example

[0052] FIG. 1 illustrates the results of examining the influence of the amount of Sn+Sb (the total amount of Sn and Sb) in the raw material on the magnetic flux density B.sub.8 of the product sheet. As illustrated in FIG. 1, by appropriately limiting the amount of Sn+Sb in the raw material while setting S and/or Se to 0.005% or more and 0.010% or less in total, the magnetic flux density was improved. In particular, by limiting the total amount of Sn and/or Sb to 0.005% or more and 1.000% or less, a magnetic flux density B.sub.8 of 1.88 T or more was obtained. Moreover, by limiting the total amount of Sn and/or Sb to 0.020% or more and 0.300% or less, a magnetic flux density B.sub.8 of 1.900 T or more was obtained.

[0053] The reasons why the magnetic flux density of the product sheet was improved by appropriately limiting the amount of Sn+Sb in the raw material while setting S and/or Se to 0.005% or more and 0.010% or less in total are not exactly clear, but we consider the reasons as follows. S and Se, by combined use of the grain boundary segregation effect by solute S and Se content and the precipitates such as MnS and MnSe or Cu.sub.2S and Cu.sub.2Se, can enhance the normal grain growth inhibiting effect and sharpen the orientation of Goss grains growing during secondary recrystallization, so that the magnetic properties of the product which have been a problem with the low-temperature slab heating method can be improved significantly. Moreover, Sn and Sb are known as grain boundary segregation elements, and contribute to the normal grain growth inhibiting capability. Furthermore, in the case where a large amount of S and/or Se is contained as in the present disclosure, the solute amount of S and/or Se increases in addition to the precipitate amount of sulfide and selenide. An increase in the solute amount of S and/or Se leads to an increase in the grain boundary segregation amount of S and/or Se. This creates a state (i.e. co-segregation) in which the grain boundary segregation of Sn and Sb is facilitated, as a result of which the effect of grain boundary segregation increases.

[0054] The basic components according to the present disclosure have been described above. The balance other than the above-mentioned components is Fe and inevitable impurities. In the present disclosure, the following elements may also be optionally added as appropriate.

[0055] Ni in an amount of 0.005% or more and 1.5% or less

[0056] Ni is an austenite forming element, and accordingly is a useful element in improving the texture of the hot rolled sheet and improving the magnetic properties through austenite transformation. If the amount of Ni is less than 0.005%, the effect of improving the magnetic properties is low. If the amount of Ni is more than 1.5%, workability decreases, and so sheet passing performance decreases. Besides, secondary recrystallization becomes unstable, which causes degradation in magnetic properties. The amount of Ni is therefore in a range of 0.005% to 1.5%.

[0057] Cu in an amount of 0.005% or more and 1.5% or less, Cr in an amount of 0.005% or more and 0.1% or less, P in an amount of 0.005% or more and 0.5% or less, Mo in an amount of 0.005% or more and 0.5% or less, Ti in an amount of 0.0005% or more and 0.1% or less, Nb in an amount of 0.0005% or more and 0.1% or less, V in an amount of 0.0005% or more and 0.1% or less, B in an amount of 0.0002% or more and 0.0025% or less, Bi in an amount of 0.005% or more and 0.1% or less, Te in an amount of 0.0005% or more and 0.01% or less, Ta in an amount of 0.0005% or more and 0.01% or less

[0058] Cu, Cr, P, Mo, Ti, Nb, V, B, Bi, Te, and Ta are each a useful element in magnetic property improvement. If the content is less than the lower limit of the corresponding range mentioned above, the magnetic property improving effect is low. If the content is more than the upper limit of the corresponding range mentioned above, secondary recrystallization becomes unstable, which causes degradation in magnetic properties. Accordingly, in the case of adding any of these elements, the amount of Cu is in a range of 0.005% or more and 1.5% or less, the amount of Cr is in a range of 0.005% or more and 0.1% or less, the amount of P is in a range of 0.005% or more and 0.5% or less, the amount of Mo is in a range of 0.005% or more and 0.5% or less, the amount of Ti is in a range of 0.0005% or more and 0.1% or less, the amount of Nb is in a range of 0.0005% or more and 0.1% or less, the amount of V is in a range of 0.0005% or more and 0.1% or less, the amount of B is in a range of 0.0002% or more and 0.0025% or less, the amount of Bi is in a range of 0.005% or more and 0.1% or less, the amount of Te is in a range of 0.0005% or more and 0.01% or less, and the amount of Ta is in a range of 0.0005% or more and 0.01% or less.

[0059] The present disclosure provides a method that combines a minute amount of precipitate and a grain boundary segregation element, which can be referred to as subtle inhibition control (SIC) method. The SIC method is more advantageous than the conventional inhibitor technique or inhibitorless technique, as it can simultaneously achieve the low-temperature slab heating and the normal grain growth inhibiting effect.

[0060] It is considered that, in the case of being redissolved in the slab heating, S and Se precipitate as fine MnS and MnSe during hot rolling, and contribute to enhanced normal grain growth inhibiting capability. If the total amount of S and/or Se is less than 0.005%, this effect is insufficient, so that the magnetic property improving effect cannot be achieved. If the total amount of S and/or Se is more than 0.010%, the redissolution in the low-temperature slab heating at 1300? C. or less is insufficient, and the normal grain growth inhibiting capability decreases rapidly. This causes a secondary recrystallization failure.

[0061] A production method according to the present disclosure is described below.

[Heating]

[0062] A steel slab having the above-mentioned chemical composition is subjected to slab heating. The slab heating temperature is 1300? C. or less. Heating at more than 1300? C. requires the use of not ordinary gas heating but a special heating furnace such as induction heating, and so is disadvantageous in terms of cost, productivity, yield rate, and the like.

[0063] [Hot Rolling]

[0064] After this, hot rolling is performed. The hot rolling conditions are, for example, a rolling reduction of 95% or more and a sheet thickness after hot rolling of 1.5 mm to 3.5 mm. The rolling finish temperature is desirably 800? C. or more. The coiling temperature after the hot rolling is desirably about 500? C. to 700? C.

[0065] [Hot Band Annealing]

[0066] After the hot rolling, hot band annealing is optionally performed to improve the texture of the hot rolled sheet. The hot band annealing is preferably performed under the conditions of a soaking temperature of 800? C. or more and 1200? C. or less and a soaking time of 2 s or more and 300 s or less.

[0067] If the soaking temperature in the hot band annealing is less than 800? C., the texture of the hot rolled sheet is not completely improved, and non-recrystallized parts remain, so that desired texture may be unable to be obtained. If the soaking temperature is more than 1200? C., the dissolution of AlN, MnSe, and MnS proceeds, and the inhibiting capability of the inhibitors in the secondary recrystallization process is insufficient, as a result of which secondary recrystallization is suspended. This causes degradation in magnetic properties. Accordingly, the soaking temperature in the hot band annealing is preferably 800? C. or more and 1200? C. or less.

[0068] If the soaking time is less than 2 s, non-recrystallized parts remain because of the short high-temperature holding time, so that desired texture may be unable to be obtained. If the soaking time is more than 300 s, the dissolution of AlN, MnSe, and MnS proceeds, and the above-mentioned effect of N, sol.Al, Sn+Sb, and S+Se added in minute amounts decreases, as a result of which the texture of the cold rolled sheet becomes non-uniform. This causes degradation in the magnetic properties of the secondary recrystallization annealed sheet. Accordingly, the soaking time in the hot band annealing is preferably 2 s or more and 300 s or less.

[0069] [Cold Rolling]

[0070] After the hot rolling or the hot band annealing, the steel sheet is subjected to cold rolling twice or more with intermediate annealing performed therebetween, to a final sheet thickness. In this case, the intermediate annealing is preferably performed with a soaking temperature of 800? C. or more and 1200? C. or less and a soaking time of 2 s or more and 300 s or less, for the same reasons as in the hot band annealing.

[0071] In the cold rolling, by setting the rolling reduction in final cold rolling to 80% or more and 95% or less, better texture of the primary recrystallization annealed sheet can be obtained. It is also effective to perform the rolling with the rolling temperature increased to 100? C. to 250? C., or perform aging treatment once or more in a range of 100? C. to 250? C. during the cold rolling, in terms of developing Goss texture.

[0072] [Primary Recrystallization Annealing]

[0073] After the cold rolling, the cold rolled sheet is subjected to primary recrystallization annealing preferably at a soaking temperature of 700? C. or more and 1000? C. or less. The primary recrystallization annealing may be performed in, for example, a wet hydrogen atmosphere to additionally obtain the effect of decarburization of the steel sheet. If the soaking temperature in the primary recrystallization annealing is less than 700? C., non-recrystallized parts remain, and desired texture may be unable to be obtained. If the soaking temperature is more than 1000? C., there is a possibility that the secondary recrystallization of Goss orientation grains occurs. Accordingly, the soaking temperature in the primary recrystallization annealing is preferably 700? C. or more and 1000? C. or less. In the primary recrystallization annealing, the average heating rate in a temperature range of 500? C. to 700? C. is preferably 50? C./s or more.

[0074] [Nitriding Treatment]

[0075] Further, in the present disclosure, nitriding treatment may be applied in any stage between the primary recrystallization annealing and the secondary recrystallization annealing. As the nitriding treatment, any of the known techniques such as performing gas nitriding by heat treatment in an ammonia atmosphere after the primary recrystallization annealing, performing salt bath nitriding by heat treatment in a salt bath, performing plasma nitriding, adding nitride to the annealing separator, and using a nitriding atmosphere as the secondary recrystallization annealing atmosphere, may be used.

[0076] [Secondary Recrystallization Annealing]

[0077] Subsequently, an annealing separator mainly composed of MgO is optionally applied to the steel sheet surface, and then the steel sheet is subjected to secondary recrystallization annealing. Here, one or more selected from sulfide, sulfate, selenide, and selenate may be added to the annealing separator. These additives dissolve during the secondary recrystallization annealing, and then causes sulfurizing and selenizing in the steel, to thereby provide an inhibiting effect. The annealing conditions of the secondary recrystallization annealing are not limited, and conventionally known annealing conditions may be used. By using a hydrogen atmosphere as the annealing atmosphere, the effect of purification annealing can also be achieved. Subsequently, after application of insulating coating and execution of flattening annealing, a desired grain-oriented electrical steel sheet is obtained. The production conditions in the application of insulating coating and the flattening annealing are not limited, and conventional methods may be used.

[0078] The grain-oriented electrical steel sheet produced according to the above-mentioned conditions has a very high magnetic flux density as well as low iron loss properties after the secondary recrystallization. A high magnetic flux density means that the crystal grains have preferentially grown only in the Goss orientation and its vicinity during the secondary recrystallization process. In the Goss orientation and its vicinity, the growth rate of secondary recrystallized grains is higher. Therefore, an increase in magnetic flux density indicates that the secondary recrystallized grain size is potentially coarse. This is advantageous in terms of reducing hysteresis loss, but disadvantageous in terms of reducing eddy current loss.

[0079] [Magnetic Domain Refining Treatment]

[0080] To solve such mutually contradictory phenomena against the ultimate goal of iron loss reduction, it is preferable to perform magnetic domain refining treatment. By performing appropriate magnetic domain refining treatment, the disadvantageous eddy current loss caused by the coarsening of secondary recrystallized grains is reduced, and together with the hysteresis loss reduction, significantly low iron loss properties can be obtained.

[0081] As the magnetic domain refining treatment, any known heat resistant or non-heat resistant magnetic domain refining treatment may be used. With the use of a method of irradiating the steel sheet surface after the secondary recrystallization annealing with an electron beam or a laser, the magnetic domain refining effect can spread to the inside of the steel sheet in the sheet thickness direction, and thus iron loss can be significantly reduced as compared with other magnetic domain refining treatment such as an etching method.

[0082] The other production conditions may comply with typical grain-oriented electrical steel sheet production methods.

EXAMPLES

Example 1

[0083] Steel slabs with a thickness of 220 mm having the respective chemical compositions listed in Table 2 were each heated to 1250? C., and then hot rolled to a thickness of 2.7 mm. After this, the hot rolled sheet was hot band annealed at 1020? C. for 60 s, and then cold rolled to a thickness of 0.27 mm. The cold rolled sheet was then subjected to primary recrystallization annealing at 840? C. for 120 s. The heating rate from 500? C. to 700? C. in the primary recrystallization annealing was 100? C./s.

[0084] Subsequently, an annealing separator mainly composed of MgO was applied to the steel sheet surface, and then the steel sheet was subjected to secondary recrystallization annealing serving also as purification annealing at 1200? C. for 10 h. Following this, a phosphate-based insulating tension coating was applied and baked on the steel sheet, and flattening annealing was performed for the purpose of flattening the steel strip, to obtain a product.

[0085] The results of examining the magnetic properties of each product obtained in this way are listed in Table 2.

TABLE-US-00002 TABLE 2 Secondary recrystallization Chemical composition (mass %) annealed sheet sol. B.sub.8 W.sub.17/50 No. Si C Mn Al N S Se Sn Sb Others S + Se Sn + Sb (T) (W/kg) Remarks 1 1.82 0.015 0.09 0.008 0.003 0.006 0 0 0.080 0.006 0.080 1.866 1.292 Comparative Example 2 8.55 0.044 0.10 0.009 0.004 0.005 0.003 0.120 0 0.008 0.120 1.810 0.953 Comparative Example 3 3.22 0.001 0.09 0.008 0.004 0.007 0.001 0.110 0.090 0.008 0.200 1.843 1.229 Comparative Example 4 3.30 0.089 0.10 0.008 0.005 0.006 0 0.090 0.110 0.006 0.200 1.865 1.155 Comparative Example 5 3.29 0.050 0.01 0.006 0.003 0.005 0 0.090 0.100 0.005 0.190 1.857 1.132 Comparative Example 6 3.36 0.056 0.56 0.005 0.004 0.006 0.002 0.110 0.090 0.008 0.200 1.826 1.333 Comparative Example 7 3.43 0.042 0.09 0.010 0.005 0.006 0 0.060 0.060 0.006 0.120 1.638 2.117 Comparative Example 8 3.33 0.051 0.08 0.002 0.004 0.005 0.001 0.060 0.050 0.006 0.110 1.588 2.430 Comparative Example 9 3.50 0.053 0.09 0.009 0.006 0.002 0.008 0 0.050 0.010 0.050 1.674 2.005 Comparative Example 10 7.43 0.078 0.41 0.008 0.004 0.007 0.001 0 0.006 Ni: 0.007, Bi: 0.009 0.008 0.006 1.902 0.872 Example 11 3.19 0.022 0.09 0.008 0.004 0.004 0.004 0.004 0.001 Cu: 0.005, Ti: 0.011, 0.008 0.005 1.917 0.946 Example Nb: 0.089 12 2.42 0.033 0.21 0.009 0.003 0.008 0.001 0.001 0.066 Cr: 0.006, Mo: 0.47, 0.009 0.067 1.923 0.981 Example B: 0.0023 13 3.25 0.051 0.09 0.008 0.004 0.007 0 0.080 0.001 Cu: 0.07, Cr: 0.09, 0.007 0.081 1.926 0.902 Example Ti: 0.0011, Bi: 0.030 14 4.13 0.046 0.08 0.007 0.004 0.006 0.002 0.041 0.039 P: 0.008, V: 0.094, 0.008 0.080 1.920 0.901 Example Te: 0.0006, Ta: 0.009 15 3.36 0.042 0.08 0.006 0.004 0.004 0.004 0.025 0.053 Cu: 0.12, Cr: 0.053, 0.008 0.078 1.932 0.911 Example Mo: 0.036, Ti: 0.0008, Nb: 0.0022 16 3.88 0.053 0.09 0.008 0.003 0.002 0.006 0.071 0.001 Mo: 0.007, V: 0.0006, 0.008 0.072 1.933 0.909 Example Bi: 0.095 17 4.40 0.048 0.07 0.008 0.004 0.006 0 0.044 0.060 Ni: 1.3, Cu: 1.4, 0.006 0.104 1.924 0.890 Example Nb: 0.006, B: 0.0003 18 3.52 0.030 0.08 0.007 0.004 0.006 0.001 0.001 0.071 Cu: 0.09, Cr: 0.048, 0.007 0.072 1.926 0.924 Example P: 0.067, Mo: 0.013, Ti: 0.0014 19 3.44 0.049 0.16 0.009 0.004 0.005 0.004 0.001 0.052 Cu: 0.11, Cr: 0.098, 0.009 0.053 1.935 0.897 Example Mo: 0.025, B: 0.0012, Te: 0.094 20 3.11 0.062 0.03 0.006 0.003 0.002 0.007 0.160 0.077 Ni: 0.13, P: 0.45, 0.009 0.237 1.927 0.929 Example Ti: 0.096, Ta: 0.0006 21 3.28 0.004 0.12 0.007 0.004 0.002 0.006 0.110 0.120 P: 0.022, Ti: 0.0011, 0.008 0.230 1.925 0.911 Example V: 0.014, Te: 0.008 22 3.39 0.040 0.11 0.008 0.004 0.005 0.003 0.160 0.092 Mo: 0.067, Nb: 0.0034, 0.008 0.252 1.937 0.909 Example Ta: 0.0077 23 3.70 0.057 0.10 0.009 0.004 0.004 0.001 0.220 0.100 Ni: 0.22, Cu: 0.12, 0.005 0.320 1.912 0.928 Example Mo: 0.078, Ti: 0.0017 24 3.19 0.041 0.08 0.006 0.004 0.006 0 0.360 0.210 Cr: 0.09, Ti: 0.0009, 0.006 0.570 1.905 0.967 Example Bi: 0.022

[0086] As shown in Table 2, by appropriately limiting the amount of Sn+Sb in the raw material while setting S and/or Se to 0.005% or more and 0.010% or less in total, the magnetic flux density was improved. In particular, by limiting the total amount of Sn and/or Sb to 0.005% or more and 1.000% or less, a magnetic flux density B.sub.8 of 1.900 T or more was obtained. Moreover, by limiting the total amount of Sn and/or Sb to 0.020% or more and 0.300% or less, a magnetic flux density B.sub.8 of 1.920 T or more was obtained.

Example 2

[0087] The steel slabs of Nos. 13 and 18 in Table 2 were each heated to 1230? C., and then hot rolled to a thickness of 2.7 mm. The hot rolled sheet was then hot band annealed at 1000? C. for 60 s, and subsequently subjected to the first cold rolling to an intermediate thickness of 2.0 mm. After intermediate annealing at 1040? C. for 60 s, the steel sheet was subjected to the second cold rolling to a thickness of 0.23 mm. The cold rolled sheet was then subjected to primary recrystallization annealing at 820? C. for 120 s. The heating rate from 500? C. to 700? C. in the primary recrystallization annealing was 150? C./s. Following this, the nitriding treatment and the addition of sulfate to the annealing separator were examined under the conditions listed in Table 3. As the nitriding treatment, gas nitriding treatment was performed on the primary recrystallization annealed sheet at 750? C. for 30 s and at 950? C. for 30 s in a gas atmosphere containing ammonia. The amount of nitrogen in the steel sheet after subjection to the nitriding treatment is listed in Table 3. As the addition of sulfate to the annealing separator, an annealing separator containing MgO and MgSO.sub.4 in an amount of 10 parts by mass with respect to MgO in an amount of 100 parts by mass was applied to the steel sheet surface. After this, the steel sheet was subjected to secondary recrystallization annealing also serving as purification annealing at 1180? C. for 50 h. Subsequently, a phosphate-based insulation tension coating was applied and baked on the steel sheet, and flattening annealing was performed for the purpose of flattening the steel strip, to obtain a product sheet.

[0088] The results of examining the magnetic properties of each product sheet obtained in this way are listed in Table 3.

TABLE-US-00003 TABLE 3 Secondary recrystallization Nitrided annealed sheet sheet N B.sub.8 W.sub.17/50 ID Nitriding treatment (mass %) Annealing separator (T) (W/kg) Remarks 13-a None 0.004 100: MgO 1.921 0.840 Example 13-b 0.004 100: MgO + 10: MgSO.sub.4 1.941 0.807 Example 13-c 750? C. ? 30 s 0.023 100: MgO 1.943 0.811 Example 13-d 0.025 100: MgO + 10: MgSO.sub.4 1.947 0.798 Example 13-e 950? C. ? 30 s 0.027 100: MgO 1.942 0.809 Example 13-f 0.025 100: MgO + 10: MgSO.sub.4 1.947 0.800 Example 18-a None 0.004 100: MgO 1.922 0.829 Example 18-b 0.004 100: MgO + 10: MgSO.sub.4 1.942 0.782 Example 18-c 750? C. ? 30 s 0.022 100: MgO 1.940 0.784 Example 18-d 0.024 100: MgO + 10: MgSO.sub.4 1.944 0.776 Example 18-e 950? C. ? 30 s 0.025 100: MgO 1.941 0.779 Example 18-f 0.026 100: MgO + 10: MgSO.sub.4 1.945 0.775 Example

[0089] As shown in Table 3, by limiting the total amount of S and/or Se to 0.005% or more and 0.010% or less and the total amount of Sn and/or Sb to 0.020% or more and 0.300% or less, a magnetic flux density B.sub.8 of 1.920 T or more was obtained. In addition, by performing the nitriding treatment on the primary recrystallization annealed sheet or adding sulfate to the annealing separator, a magnetic flux density B.sub.8 of 1.940 T or more was obtained.

Example 3

[0090] For the samples of Nos. 13-b, 13-c, 18-b, and 18-c in Table 3, an experiment for determining the effect of magnetic domain refining treatment listed in Table 5 was conducted. Etching was performed to form grooves of 80 ?m in width, 15 ?m in depth, and 5 mm in rolling direction interval in the direction orthogonal to the rolling direction on one surface of the cold rolled steel sheet. An electron beam was continuously applied to one surface of the steel sheet after subjection to the flattening annealing in the direction orthogonal to the rolling direction, under the conditions of an acceleration voltage of 80 kV, an irradiation interval of 5 mm, and a beam current of 3 mA. A continuous laser was continuously applied to one surface of the steel sheet after subjection to the flattening annealing in the direction orthogonal to the rolling direction, under the conditions of a beam diameter of 0.3 mm, a power of 200 W, a scanning rate of 100 m/s, and an irradiation interval of 5 mm.

[0091] The results of examining the magnetic properties of each product obtained in this way are listed in Table 4.

TABLE-US-00004 TABLE 4 Secondary recrystallization Nitrided annealed sheet sheet N Magnetic domain B.sub.8 W.sub.17/50 ID Nitriding treatment (mass %) Annealing separator refining treatment (T) (W/kg) Remarks 13-b None 0.004 100: MgO + 10: MgSO.sub.4 None 1.941 0.807 Example 13-b-X 0.004 Etching groove 1.914 0.726 Example 13-b-Y 0.004 Electron beam 1.940 0.698 Example 13-b-Z 0.004 Continuous laser 1.939 0.697 Example 13-c 750? C. ? 30 s 0.023 100: MgO None 1.943 0.811 Example 13-c-X 0.025 Etching groove 1.913 0.724 Example 13-c-Y 0.023 Electron beam 1.942 0.700 Example 13-c-Z 0.024 Continuous laser 1.942 0.699 Example 18-b None 0.004 100: MgO + 10: MgSO.sub.4 None 1.942 0.782 Example 18-b-X 0.004 Etching groove 1.909 0.704 Example 18-b-Y 0.004 Electron beam 1.941 0.684 Example 18-b-Z 0.004 Continuous laser 1.941 0.688 Example 18-c 750? C. ? 30 s 0.022 100: MgO None 1.940 0.784 Example 18-c-X 0.025 Etching groove 1.907 0.702 Example 18-c-Y 0.023 Electron beam 1.939 0.685 Example 18-c-Z 0.024 Continuous laser 1.938 0.689 Example

[0092] As shown in Table 4, by performing the magnetic domain refining treatment, better iron loss properties were obtained. In detail, excellent iron loss properties equivalent to those of a high-temperature slab heated material, i.e. an iron loss W.sub.17/50 of 0.70 W/kg or less after the magnetic domain refining treatment by an electron beam or a continuous laser, can be obtained by the production method of low cost and high productivity according to the present disclosure.

INDUSTRIAL APPLICABILITY

[0093] According to the present disclosure, by controlling minute amount inhibitors, the normal grain growth inhibiting capability is enhanced and the orientation of Goss grains growing during secondary recrystallization is sharpened, with it being possible to significantly improve the magnetic properties of the product which have been a problem with the low-temperature slab heating method. In particular, even for a thin steel sheet with a sheet thickness of 0.23 mm which has been considered difficult to increase in magnetic flux density, excellent magnetic properties, i.e. a magnetic flux density B.sub.g of 1.92 T or more after secondary recrystallization annealing, can be stably obtained throughout the coil length.