METHOD OF MANUFACTURING GRAIN-ORIENTED ELECTRICAL STEEL SHEET

Abstract

A steel slab having a composition not containing an inhibitor component further contains, in mass %, at least one selected from: Sn: 0.010% to 0.200%; Sb: 0.010% to 0.200%; Mo: 0.010% to 0.150%; and P: 0.010% to 0.150%, and annealing that satisfies a relationship Td≧Tf is performed, where Td (° C.) is a highest temperature at which the steel sheet is annealed in decarburization annealing and Tf (° C.) is a highest temperature before secondary recrystallization of the steel sheet starts in final annealing. Thus, a grain-oriented electrical steel sheet with significantly reduced magnetic property scattering in a coil is obtained without using an inhibitor component.

Claims

1. A method of manufacturing a grain-oriented electrical steel sheet, the method comprising: reheating a steel slab in a temperature range of 1300° C. or less, the steel slab having a composition that contains, in mass % or mass ppm, C: 0.002% to 0.08%, Si: 2.0% to 8.0%, Mn: 0.005% to 1.0%, N: less than 50 ppm, S: less than 50 ppm, Se: less than 50 ppm, and sol.Al: less than 100 ppm, with a balance being Fe and incidental impurities; hot rolling the reheated 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, to form a cold rolled steel sheet having a final sheet thickness; performing decarburization annealing that also serves as primary recrystallization annealing, on the cold rolled steel sheet; applying an annealing separator to a surface of the steel sheet after the decarburization annealing; and performing final annealing on the steel sheet with the annealing separator applied, wherein the steel slab further contains, in mass %, at least one selected from: Sn: 0.010% to 0.200%; Sb: 0.010% to 0.200%; Mo: 0.010% to 0.150%; and P: 0.010% to 0.150%, and a relationship Td≧Tf is satisfied, where Td (° C.) is a highest temperature at which the steel sheet is annealed in the decarburization annealing and Tf (° C.) is a highest temperature before secondary recrystallization of the steel sheet starts in the final annealing.

2. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the steel sheet is retained at a temperature of Td (° C.) or less for 20 hours or more in the final annealing.

3. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the steel sheet is in a temperature range of 400° C. to 700° C. in the final annealing for a residence time of 10 hours or more.

4. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein an annealing atmosphere before the secondary recrystallization starts in the final annealing is a N.sub.2 atmosphere.

5. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

6. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein the steel sheet is in a temperature range of 400° C. to 700° C. in the final annealing for a residence time of 10 hours or more.

7. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein an annealing atmosphere before the secondary recrystallization starts in the final annealing is a N.sub.2 atmosphere.

8. The method of manufacturing a grain-oriented electrical steel sheet according to claim 3, wherein an annealing atmosphere before the secondary recrystallization starts in the final annealing is a N.sub.2 atmosphere.

9. The method of manufacturing a grain-oriented electrical steel sheet according to claim 6, wherein an annealing atmosphere before the secondary recrystallization starts in the final annealing is a N.sub.2 atmosphere.

10. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

11. The method of manufacturing a grain-oriented electrical steel sheet according to claim 3, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

12. The method of manufacturing a grain-oriented electrical steel sheet according to claim 4, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

13. The method of manufacturing a grain-oriented electrical steel sheet according to claim 6, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

14. The method of manufacturing a grain-oriented electrical steel sheet according to claim 7, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

15. The method of manufacturing a grain-oriented electrical steel sheet according to claim 8, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

16. The method of manufacturing a grain-oriented electrical steel sheet according to claim 9, wherein the steel slab further contains, in mass % or mass ppm, at least one selected from: Ni: 0.010% to 1.50%; Cr: 0.01% to 0.50%; Cu: 0.01% to 0.50%; Bi: 0.005% to 0.50%; Te: 0.005% to 0.050%; and Nb: 10 ppm to 100 ppm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In the accompanying drawings:

[0056] FIG. 1 is a diagram illustrating the influence of the latter stage temperature of decarburization annealing and the first stage temperature of final annealing on the magnetic property scattering in the coil; and

[0057] FIG. 2 is a diagram illustrating the influence of the difference in raw material component on the magnetic property scattering in the coil.

DETAILED DESCRIPTION

[0058] Detailed description is given below.

[0059] The reasons for limiting the composition according to the present disclosure are described first.

[0060] C: 0.002 Mass % to 0.08 Mass %

[0061] If the C content is less than 0.002 mass %, the grain boundary strengthening effect by C is poor, and defects which hamper manufacture, such as slab cracking, appear. If the C content is more than 0.08 mass %, it is difficult to reduce, by decarburization annealing, the content to 0.005 mass % or less that causes no magnetic aging. The C content is therefore in the range of 0.002 mass % to 0.08 mass %. The C content is preferably 0.010 mass % or more. The C content is preferably 0.08 mass % or less.

[0062] Si: 2.0 Mass % to 8.0 Mass %

[0063] Si is an element necessary to increase the specific resistance of the steel and reduce iron loss. This effect is insufficient if the Si content is less than 2.0 mass %. If the Si content is more than 8.0 mass %, workability decreases and manufacture by rolling is difficult. The Si content is therefore in the range of 2.0 mass % to 8.0 mass %. The Si content is preferably 2.5 mass % or more. The Si content is preferably 4.5 mass % or less.

[0064] Mn: 0.005 Mass % to 1.0 Mass %

[0065] Mn is an element necessary to improve the hot workability of the steel. This effect is insufficient if the Mn content is less than 0.005 mass %. If the Mn content is more than 1.0 mass %, the magnetic flux density of the product sheet decreases. The Mn content is therefore in the range of 0.005 mass % to 1.0 mass %. The Mn content is preferably 0.02 mass % or more. The Mn content is preferably 0.20 mass % or less.

[0066] The present disclosure relates to a technique not using an inhibitor, as mentioned above. Accordingly, in the steel raw material in the present disclosure, the content of each of N, S, and Se as an inhibitor forming component is limited to less than 50 mass ppm, and the content of sol.Al as an inhibitor forming component is limited to 100 mass ppm or less.

[0067] In the present disclosure, it is essential to contain, as a grain boundary segregation element, at least one selected from: Sn: 0.010 mass % to 0.200 mass %; Sb: 0.010 mass % to 0.200 mass %; Mo: 0.010 mass % to 0.150 mass %; and P: 0.010 mass % to 0.150 mass %, to enhance the normal grain growth suppression effect by the grain boundary segregation element during final annealing.

[0068] If the content of any of Sn, Sb, Mo, and P is less than the aforementioned lower limit, the magnetic property scattering reduction effect is poor. If the content of any of Sn, Sb, Mo, and P is more than the aforementioned upper limit, the magnetic flux density decreases and the magnetic property degrades.

[0069] The balance other than the aforementioned components in the grain-oriented electrical steel sheet in the present disclosure is Fe and incidental impurities, but the following other elements may be contained as appropriate.

[0070] At least one selected from: Ni: 0.010 mass % to 1.50 mass %; Cr: 0.01 mass % to 0.50 mass %; Cu: 0.01 mass % to 0.50 mass %; Bi: 0.005 mass % to 0.50 mass %; Te: 0.005 mass % to 0.050 mass %; and Nb: 10 mass ppm to 100 mass ppm may be added. If the content of any of these elements is less than the lower limit, the iron loss reduction effect is poor. If the content of any of these elements is more than the upper limit, the magnetic flux density decreases and the magnetic property degrades.

[0071] The following describes a method of manufacturing a grain-oriented electrical steel sheet according to the present disclosure.

[0072] In the present disclosure, molten steel prepared to have the aforementioned predetermined components may be made into a slab by typical ingot casting or continuous casting, or made into a thin slab or thinner cast steel with a thickness of 100 mm or less by direct casting. Of the aforementioned components, components difficult to be added in an intermediate step are desirably added in the molten steel stage.

[0073] The slab is heated and hot rolled by a typical method. Here, since the chemical composition in the present disclosure does not need high-temperature annealing for dissolving an inhibitor, low temperature of 1300° C. or less is cost advantageous. A desirable slab heating temperature is 1250° C. or less.

[0074] Next, hot band annealing is desirably performed to attain favorable magnetic property. The hot band annealing temperature is preferably 800° C. or more. The hot band annealing temperature is preferably 1100° C. or less. If the hot band annealing temperature is more than 1200° C., the grain size coarsens excessively, which is significantly disadvantageous in realizing primary recrystallized texture of uniformly-sized grains. The hot band annealing may be omitted.

[0075] Next, cold rolling is performed once or twice or more with intermediate annealing in between, to form a cold rolled steel sheet.

[0076] The intermediate annealing temperature is preferably 900° C. or more. The intermediate annealing temperature is preferably 1200° C. or less. If the temperature is less than 900° C., the recrystallized grains become fine, which reduces Goss nuclei in primary recrystallized texture and degrades magnetic property. If the temperature is more than 1200° C., the grain size coarsens excessively as in the hot band annealing, which is significantly disadvantageous in realizing primary recrystallized texture of uniformly-sized grains.

[0077] In final cold rolling, it is effective to increase the cold rolling temperature to 100° C. to 300° C. and also perform aging treatment in the range of 100° C. to 300° C. once or more during the cold rolling, in order to change the recrystallized texture and improve the magnetic property.

[0078] After the cold rolling, decarburization annealing is performed.

[0079] As the decarburization annealing in the present disclosure, annealing in the temperature range of 800° C. or more and 900° C. or less is effective in terms of efficient decarburization. Moreover, in the present disclosure, the decarburization annealing temperature needs to be higher than the temperature before secondary recrystallization in final annealing, as mentioned above. To realize efficient decarburization, however, it is desirable to divide the decarburization annealing into two stages, in which annealing is performed in a temperature range that eases decarburization in the first stage and annealing is performed at higher temperature in the latter stage. Here, the annealing at higher temperature is intended to control the primary recrystallized grain size, and so the annealing atmosphere is not particularly defined. The atmosphere may be a wet atmosphere or a dry atmosphere. In the present disclosure, the highest temperature at which the steel sheet is annealed in the decarburization annealing is defined as Td (° C.).

[0080] Following this, an annealing separator mainly containing MgO is applied to the steel sheet, and then the steel sheet is subjected to final annealing to develop secondary recrystallized texture and also form a forsterite film. In the present disclosure, the temperature before starting the secondary recrystallization in the final annealing needs to be lower than the highest temperature Td (° C.) in the decarburization annealing. Here, since there is typically an appropriate temperature for secondary recrystallization, it is effective to control the decarburization annealing temperature rather than controlling the final annealing temperature. In the present disclosure, the highest temperature before the secondary recrystallization of the steel sheet starts in the final annealing is defined as Tf (° C.).

[0081] The main feature in the present disclosure is to perform the decarburization annealing and the final annealing under a condition that Td (° C.) and Tf (° C.) satisfy the relationship Td≧Tf.

[0082] The final annealing is desirably performed at 800° C. or more, to develop secondary recrystallization. Moreover, retention for 20 hours or more in a temperature range appropriate for secondary recrystallization is desirable as there is no need to take into account the variation in the latent period of secondary recrystallization.

[0083] In the present disclosure, the steel sheet is in the temperature range of 400° C. to 700° C. especially during the temperature increase in the final annealing for a residence time of desirably 10 hours or more, to facilitate grain boundary segregation. In addition, the annealing atmosphere before the start of secondary recrystallization is desirably a N.sub.2 atmosphere, as a slight amount of nitride forms in the steel and inhibits normal grain growth.

[0084] The N.sub.2 atmosphere mentioned here may be any atmosphere whose main component is N.sub.2. In detail, any atmosphere containing 60 vol % or more N.sub.2 in partial pressure ratio is applicable. To form a forsterite film, the final annealing temperature after the start of secondary recrystallization is desirably increased to about 1200° C.

[0085] After the final annealing, washing, brushing, or pickling is useful to remove the attached annealing separator.

[0086] It is effective to further perform flattening annealing to adjust the shape, for iron loss reduction. In the case of using the steel sheet in a stacked state, it is effective to apply an insulation coating to the steel sheet surface before or after the flattening annealing, in order to improve iron loss. Applying such a coating that imparts tension to the steel sheet is also useful for iron loss reduction.

[0087] A method of forming a coating by depositing an inorganic substance onto the steel sheet surface layer by tension coating application through a binder, physical vapor deposition, or chemical vapor deposition is desirable as coating adhesion is excellent and a considerable iron loss reduction effect is achieved.

[0088] In addition, magnetic domain refining treatment may be performed to further reduce iron loss. A typical method such as grooving the steel sheet after final annealing, introducing linear thermal strain or impact strain by laser, an electron beam, plasma, etc., or grooving beforehand an intermediate product such as the cold rolled steel sheet that has reached the final sheet thickness may be used.

EXAMPLES

[0089] Examples are described below.

Example 1

[0090] A steel slab containing, in mass % or mass ppm, C: 0.063%, Si: 3.33%, Mn: 0.23%, sol.Al: 84 ppm, S: 33 ppm, Se: 15 ppm, N: 14 ppm, and Sn: 0.075% with the balance being Fe and incidental impurities was manufactured by continuous casting, heated at 1200° C., and then hot rolled to a thickness of 2.7 mm. The hot rolled steel sheet was hot band annealed at 1000° C. for 30 seconds, and then cold rolled to a sheet thickness of 0.27 mm. Further, the cold rolled steel sheet was subjected to decarburization annealing, at 830° C. for 120 seconds in a wet atmosphere of 45% H.sub.2-55% N.sub.2 with a dew point of 60° C. in the first stage and at various temperatures from 820° C. to 940° C. for 10 seconds in a dry atmosphere of 45% H.sub.2-55% N.sub.2 with a dew point of −20° C. in the latter stage. Following this, an annealing separator mainly containing MgO was applied to the steel sheet. The steel sheet was then coiled, and subjected to final annealing. In the final annealing, the first stage was performed at 850° C. for 50 hours in a N.sub.2 atmosphere to start secondary recrystallization, and then the latter stage was performed at 1200° C. for 10 hours in a hydrogen atmosphere. Here, the residence time in the temperature range of 400° C. to 700° C. during the temperature increase in the first stage was controlled to 15 hours, to facilitate the segregation of the grain boundary segregation element.

[0091] The iron loss W.sub.17/50 (iron loss in the case of performing excitation of 1.7 T at a frequency of 50 Hz) of the obtained sample was measured by the method described in JIS-C-2550. The iron loss evaluation was performed for a total of five parts selected from both longitudinal ends, center, and intermediate positions between the respective ends and center of the coil, and the difference ΔW between the maximum and minimum values of the five parts was set as an index of the magnetic property scattering in the coil.

[0092] The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Latter stage temperature of decarburization Iron loss Scattering annealing W.sub.17/50 DW ° C. W/kg W/kg Remarks 820 0.933 0.047 Comparative Example 840 0.846 0.034 Comparative Example 860 0.832 0.016 Example 880 0.839 0.009 Example 900 0.829 0.011 Example 920 0.841 0.014 Example 940 0.845 0.011 Example

[0093] As is clear from the table, favorable iron loss property was attained with little magnetic property scattering in the range where the relationship Td≧Tf was satisfied according to the present disclosure.

Example 2

[0094] Each of the steel slabs having the respective chemical compositions shown in Table 2 with the balance being Fe and incidental impurities was manufactured by continuous casting, heated at 1180° C., and then hot rolled to a thickness of 2.7 mm. The hot rolled steel sheet was hot band annealed at 950° C. for 30 seconds, and then cold rolled to a sheet thickness of 1.8 mm. The cold rolled steel sheet was intermediate annealed at 1100° C. for 100 seconds, and then warm rolled at 100° C. to a sheet thickness of 0.23 mm. Further, the steel sheet was subjected to decarburization annealing, at 840° C. for 100 seconds in a wet atmosphere of 60% H.sub.2-40% N.sub.2 with a dew point of 60° C. in the first stage and at 900° C. for 10 seconds in a wet atmosphere of 60% H.sub.2-40% N.sub.2 with a dew point of 60° C. in the latter stage. Following this, an annealing separator mainly containing MgO was applied to the steel sheet. The steel sheet was then coiled, and subjected to final annealing. In the final annealing, the first stage was performed at 875° C. for 50 hours in a N.sub.2 atmosphere to start secondary recrystallization, and then the latter stage was performed at 1220° C. for 5 hours in a hydrogen atmosphere. Here, the residence time in the temperature range of 400° C. to 700° C. during the temperature increase in the first stage was controlled to 20 hours, to facilitate the segregation of the grain boundary segregation element.

[0095] The iron loss W.sub.17/50 (iron loss in the case of performing excitation of 1.7 T at a frequency of 50 Hz) of the obtained sample was measured by the method described in JIS-C-2550. The iron loss evaluation was performed for a total of five parts selected from both longitudinal ends, center, and intermediate positions between the respective ends and center of the coil, and the difference ΔW between the maximum and minimum values of the five parts was set as an index of the magnetic property scattering in the coil.

[0096] The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Iron Steel slab component loss C Si Mn N S Se sol. Al Sb Sn Mo P Others W.sub.17/50 ΔW % % % ppm ppm ppm ppm % % % % % (W/kg) (W/kg) Remarks 0.062 3.34 0.16 24 17 <5 73 — — — — — 0.857 0.033 Comparative Example 0.055 3.38 0.18 31 36 <5 80 0.068 — — — — 0.816 0.012 Example 0.035 3.36 0.18 28 33 30 80 — 0.033 — — — 0.824 0.011 Example 0.040 3.35 0.15 19 39 <5 67 — — 0.038 — — 0.825 0.011 Example 0.052 3.38 0.17 14 12 30 24 — — — 0.055 — 0.820 0.014 Example 0.056 3.32 0.16 43 43 <5 37 0.036 0.050 0.022 0.028 — 0.805 0.007 Example 0.120 3.21 0.18 13 26 <5 44 0.019 — — — — 2.005 0.285 Comparative Example 0.055 1.59 0.15 20 20 <5 27 0.055 — — — — 1.346 0.074 Comparative Example 0.049 3.35 1.31 18 28 <5 90 0.123 — — — — 1.112 0.121 Comparative Example 0.042 3.29 0.12 120 26 <5 63 0.069 — — — — 2.018 0.310 Comparative Example 0.051 3.36 0.17 47 110 <5 52 0.077 — — — — 2.352 0.325 Comparative Example 0.050 3.28 0.18 37 38 100 45 0.140 — — — — 2.329 0.418 Comparative Example 0.059 3.37 0.15 47 30 <5 160 0.055 — — — — 1.599 0.078 Comparative Example 0.055 3.37 0.17 33 35 20 43 0.045 — — 0.074 Cr: 0.07, Cu: 0.12 0.794 0.010 Example 0.024 3.35 0.17 14 39 <5 85 0.028 0.170 — — Ni: 0.18 0.801 0.013 Example 0.028 2.87 0.28 18 13 30 90 0.136 — 0.045 — Bi: 0.018, Nb: 0.0025 0.799 0.015 Example % and ppm in the table denote mass % and mass ppm.

[0097] As is clear from the table, favorable iron loss property was attained with little magnetic property scattering in the range of the chemical composition according to the present disclosure.