Method for manufacturing grain-oriented electrical steel sheet having excellent magnetic properties

1. A method for manufacturing a grain-oriented electrical steel sheet, the method comprising:

heating a grain-oriented electrical steel sheet slab; hot-rolling the heated slab; optionally annealing the hot-roiled steel sheet;

subjecting the hot-rolled steel sheet to one cold rolling or two or more cold roilings with intermediate annealing therebetween;

subjecting the cold-rolled steel sheet to primary recrystallization annealing; and subjecting the annealed steel sheet to secondary recrystallization annealing, wherein the primary recrystallization annealing sequentially comprises a first heating process of heating the steel sheet to a temperature, Ts °C., which is below a recrystallization temperature of the steel sheet, at an average heating rate of 300° C./sec or higher, wherein Ts °C. is 500-600° C., a second heating process of heating the steel sheet at a lower average heating rate than the average heating rate of the first heating process, but not lower than 100° C./sec, and a third heating process of heating the steel sheet at a lower average heating rate than the average heating rate of the second heating process.

2. The method of claim 1 , wherein the grain-oriented electrical steel sheet comprises, by wt %, Si: 2.0-4.0%, C: 0.085% or less, acid-soluble Al: 0.015-0.04%, Mn: 0.20% or less, N: 0.010% or less, S: 0.010% or less, and the balance of Fe and inevitable impurities.

3. The method of claim 2 , wherein the second heating process is performed by heating the steel sheet at an average heating rate of 100˜250° C./sec from Ts ° C. to 700° C., and the third heating process is performed by heating the steel sheet at an average heating rate of 40° C./sec or lower from 700° C. to a decarburization annealing temperature.

4. The method of claim 2 , wherein the grain-oriented electrical steel sheet has an N content of 0.006 wt % or less, and a process for increasing the content of N in the steel sheet is performed between the cold rolling and the secondary recrystallization annealing.

5. The method of claim 1 , wherein the first heating process is performed by heating the steel sheet at an average heating rate of 400° C./sec or higher, the second heating process is performed by heating the steel sheet at an average heating rate of 120˜180° C./sec from Ts ° C. to 700° C., and the third heating process is performed by heating the steel sheet at an average heating rate of 40° C./sec or lower from 700° C. to the decarburization annealing temperature.

6. The method of claim 1 , wherein the number of grains having a size of 35 μm or larger, measured when observing the cross-section of the steel sheet after the primary recrystallization annealing, but before the secondary recrystallization annealing, is less than 30.

7. The method of claim 1 , wherein the grain-oriented electrical steel sheet is heated to 1280° C. or lower before the hot rolling.

8. The method of claim 1 , wherein the volume fraction of grains having an orientation of up to 15° from the {110}<001> orientation is 2% or more when measured in a layer corresponding to ⅛ of the thickness from the surface of the steel sheet after the primary recrystallization annealing but before the secondary recrystallization annealing.

9. The method of claim 8 , wherein the volume fraction of grains having an orientation of up to 5° from the {110}<001> orientation is 0.09% or more when measured in a layer corresponding to ⅛ of the thickness from the surface of the steel sheet after the primary recrystallization annealing but before the secondary recrystallization annealing.

10. The method of claim 1 , wherein β angle as an area-weighted average of an absolute value of crystallographic orientation, measured for the steel sheet after the secondary recrystallization annealing, is controlled in the range of 1.5-2.6°, and a δ angle is controlled to 5° or less, wherein the β angle is an average angle of deviation from the {110}<001> orientation in the direction perpendicular to the rolling direction of the secondary recrystallized texture, and the δ angle is an average angle of deviation between the <001> orientation and the rolling direction in the secondary recrystallized texture.

11. The method of claim 10 , wherein the β angle measured for the steel sheet after the secondary recrystallization annealing is controlled to 2.4° or less, and the δ angle is controlled to 4.5° or less.

12. The method of claim 1 , wherein the primary recrystallization annealing is performed using a plurality of induction heating furnaces.

13. The method of claim 3 , wherein the grain-oriented electrical steel sheet has an N content of 0.006 wt % or less, and a process for increasing the content of N in the steel sheet is performed between the cold rolling and the secondary recrystallization annealing.

14. The method of claim 2 , wherein the first heating process is performed by heating the steel sheet at an average heating rate of 400° C./sec or higher, the second heating process is performed by heating the steel sheet at an average heating rate of 120˜180° C./sec from Ts ° C. to 700° C., and the third heating process is performed by heating the steel sheet at an average heating rate of 40° C./sec or lower from 700° C. to the decarburization annealing temperature.

15. The method of claim 3 , wherein the first heating process is performed by heating the steel sheet at an average heating rate of 400° C./sec or higher, the second heating process is performed by heating the steel sheet at an average heating rate of 120˜180° C./sec from Ts °C. to 700° C., and the third heating process is performed by heating the steel sheet at an average heating rate of 40° C./sec or lower from 700° C. to the decarburization annealing temperature.

16. The method of claim 2 , wherein the number of grains having a size of 35 μm or larger, measured when observing the cross-section of the steel sheet after the primary recrystallization annealing, but before the secondary recrystallization annealing, is less than 30.

17. The method of claim 3 , wherein the number of grains having a size of 35 μm or larger, measured when observing the cross-section of the steel sheet after the primary recrystallization annealing, but before the secondary recrystallization annealing, is less than 30.

18. The method of any one of claim 2 , wherein the grain-oriented electrical steel sheet is heated to 1280° C. or lower before the hot rolling.

19. The method of claim 2 , wherein the volume fraction of grains having an orientation of up to 15° from the {110}<001> orientation is 2% or more when measured in a layer corresponding to ⅛ of the thickness from the surface of the steel sheet after the primary recrystallization annealing but before the secondary recrystallization annealing.

20. The method of claim 2 , wherein a β angle as an area-weighted average of an absolute value of crystallographic orientation, measured for the steel sheet after the secondary recrystallization annealing, is controlled in the range of 1.5-2.6°, and a δ angle is controlled to 5° or less, wherein the β angle is an average angle of deviation from the {110}<001> orientation in the direction perpendicular to the rolling direction of the secondary recrystallized texture, and the δ angle is an average angle of deviation between the <001> orientation and the rolling direction in the secondary recrystallized texture.