Method of production of grain-oriented electrical steel sheet with high magnetic flux density

1. A method of production of grain-oriented electrical steel sheet comprising the steps of:

heating a silicon steel slab containing, by mass %, Si: 0.8 to 7%, C: 0.085% or less, acid soluble Al: 0.01 to 0.065%, and N: 0.012% or less at a temperature of 1280° C. or less,

then hot rolling the heated slab into a hot rolled steel strip,

annealing the hot rolled steel strip,

then cold rolling the annealed steel strip once or several times with intermediate annealing to a final steel strip thickness,

decarburization annealing the cold rolled steel strip,

then coating an annealing separator, and applying final annealing, wherein the decarburization annealed steel strip is treated to increase nitrogen in the decarburization annealed steel strip from the decarburization annealing to the start of secondary recrystallization in the final annealing,

characterized by:

performing the annealing of the hot rolled steel strip by heating to a predetermined temperature of 1000° to 1150° C. to cause recrystallization,

then annealing the recrystallized steel strip at a temperature of 850° to 1100° C. to provide a grain structure having a lamellar spacing of 20 μm or more in the annealed recrystallized steel strip, and heating the cold-rolled steel strip during decarburization annealing in a heating process consisting of only induction heating at a heating rate of 40° C./s or more in a steel strip temperature range of 550° C. to 720° C.

2. A method of production of grain-oriented electrical steel sheet comprising the steps of:

heating a silicon steel slab containing, by mass %, Si: 0.8 to 7%, C: 0.085% or less, acid soluble Al: 0.01 to 0.065%, and N: 0.012% or less at a temperature of 1280° C. or less,

then hot rolling the heated slab into a hot rolled steel strip,

annealing the hot rolled steel strip,

then cold rolling the annealed steel strip once or several times with intermediate annealing to a final steel strip thickness,

decarburization annealing the cold rolled steel strip,

then coating an annealing separator, and applying final annealing, wherein the decarburization annealed steel strip is treated to increase nitrogen in the decarburization annealed steel strip from the decarburization annealing to the start of secondary recrystallization in the final annealing, characterized by,

in the annealing of the hot rolled sheet steel strip, decarburizing the hot rolled sheet steel strip to reduce the carbon content to 0.002 to 0.02 mass % of the amount of carbon present before decarburizing by means of controlling a PH 2 O/PH 2 ratio in a moisture atmosphere or coating a decarburizing acceleration agent on the steel strip, and

after annealing the hot-rolled steel strip, subjecting the annealed hot-rolled steel strip step to decarburization annealing to provide a surface layer grain structure having a lamellar spacing of 20 μm or more, and

heating the cold rolled steel strip during decarburization annealing in a heating process consisting of only induction heating at a heating rate of 40° C./s or more in a steel strip temperature of 550° C. to 720° C.

3. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , characterized by heating the cold rolled steel strip in the decarburization annealing step at a heating rate of 50° to 250° C./s in a temperature range of a steel strip temperature of 550° C. to 720° C.

4. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , characterized by heating the cold rolled steel strip in the decarburization annealing step at a heating rate of 75° to 125° C./s in a temperature range of 550° C. to 720° C.

5. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , characterized by performing said decarburization annealing in a time interval so that the amount of oxygen of the steel strip to be decarburized becomes 2.3 g/m 2 or less and the primary recrystallization grain size becomes 15 μm or more, in a temperature range of 770° to 900° C. under the conditions where the oxidation degree (PH 2 O/PH 2 ) of the atmospheric gas is in a range of over 0.15 to 1.1.

6. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , characterized by increasing the amount of nitrogen [N] of said decarburization annealed steel strip in accordance with an amount of acid soluble Al [Al] of the steel strip so as to satisfy the formula [N]≧14/27[Al].

7. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , characterized by increasing the amount of nitrogen [N] of said decarburization annealed steel strip in accordance with an amount of acid soluble Al [Al] of the steel strip so as to satisfy the formula [N]≧2/3[Al].

8. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , wherein said annealing separator mainly comprises alumina.

9. A method of production of grain-oriented electrical steel sheet as set forth in claim 1 , characterized in that said silicon steel slab further contains, by mass %, one or more of Mn: 1% or less, Cr: 0.3% or less, Cu: 0.4% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and S and Se in a total of 0.015% or less.

10. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , characterized by heating the cold rolled steel strip in the decarburization annealing step at a heating rate of 50° to 250° C./s in a temperature range of a steel strip temperature of 550° C. to 720° C.

11. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , characterized by heating the cold rolled steel strip in the decarburization annealing step at a heating rate of 75° to 125° C./s in a temperature range of 550° C. to 720° C.

12. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , wherein the annealing separator mainly comprises alumina.

13. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , characterized in that said silicon steel slab further contains, by mass %, one or more of Mn: 1% or less, Cr: 0.3% or less, Cu: 0.4% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, and S and Se in a total of 0.015% or less.

14. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , characterized by performing said decarburization annealing in a time interval so that the amount of oxygen of the steel strip to be decarburized becomes 2.3 g/m 2 or less and the primary recrystallization grain size becomes 15 μm or more, in a temperature range of 770° to 900° C. under the conditions where the oxidation degree (PH 2 O/PH 2 ) of the atmospheric gas is in a range of over 0.15 to 1.1.

15. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , characterized by increasing the amount of nitrogen [N] of said decarburization annealed steel strip in accordance with an amount of acid soluble Al [Al] of the steel strip so as to satisfy the formula [N]≧14/27[Al].

16. A method of production of grain-oriented electrical steel sheet as set forth in claim 2 , characterized by increasing the amount of nitrogen [N] of said decarburization annealed steel strip in accordance with an amount of acid soluble Al [Al] of the steel strip so as to satisfy the formula [N]≧2/3[Al].