Method of making grain-oriented magnetic steel sheet having low iron loss

Abstract

Grain-oriented magnetic steel sheet made by the method-including hot rolling and final finish annealing, wherein (1) the O content in the steel slab is limited to up to about 30 ppm; (2) for the entire steel sheet having final thickness including an oxide film before final finish annealing, from among impurities, the Al content is limited to up to about 100 wtppm, and the respective contents of B, V, Nb, Se, S, P, and N, to up to about 50 wtppm each; and (3) during final finish annealing, the N content in the steel is, at least in the temperature region of from about 850 to 950° C., limited within the range of from about 6 to 80 wtppm.

1. A method of manufacturing a grain-oriented magnetic steel sheet, comprising the steps of hot-rolling a steel slab containing up to about 0.12 wt % C, from about 1.0 to 8.0 wt % Si, and from about 0.005 to 3.0 wt % Mn, optionally applying annealing to the resulting hot-rolled steel sheet, subjecting said annealed sheet or said hot-rolled sheet to one or more runs of cold rolling including intermediate annealing in between, optionally applying to the intermediate annealed  cold- rolled  sheet decarburization annealing, optionally coating the intermediate annealed  cold- rolled  sheet or the decarburized annealed sheet with an annealing separator and then applying final finish annealing; wherein:
 (1) the O content of said steel slab is limited to up to about 30 wtppm;  
 (2) for the entire steel sheet having final thickness including oxide film before final finish annealing, the Al content is limited to up to about 100 wtppm, and the respective contents of B, V, Nb, Se, S, P,  and N, are limited to up to about 50 wtppm each; and  
 (3) during final finish annealing, the N content in the steel is limited within a range of from about 6 to 80 ppm, at least in a temperature region of from from 850 to 950° C.  
 
     
     
       2. The method according to  claim 1 , further comprising the step of controlling the N content in the steel during final finish annealing, said controlling step comprising one or more of:
 (a) increasing the nitrogen partial pressure in the atmosphere at least in the temperature region of from about 850 to 950° C. during final finish annealing; and  
 (b) adding a nitrification accelerating agent to said annealing separator.  
 
     
     
       3. The method according to  claim 1 , wherein the maximum temperature in the final finish annealing step is up to about 1,120° C. 
     
     
       4. The method according to  claim 1 , wherein at least in a temperature region from 850° C. to the completion of secondary recrystallization of the final finish annealing, heating is conducted at a heating rate of up to about 50° C./h while imparting a temperature gradient of at least about 1.0° C./cm and up to about 10° C./cm to the steel sheet. 
     
     
       5. The method according to  claim 1 , wherein said steel slab is a direct cast steel slab and wherein said direct cast steel slab is subjected directly to hot rolling without heating the steel slab. 
     
     
       6. The method according to  claim 1 , wherein direct cast hot rolling is carried out by the use of a thin slab having a thickness of up to about 100 mm obtained from molten steel by the direct casting process, or wherein the thin slab is used as a hot-rolled steel sheet material. 
     
     
       7. The method according to  claim 1 , wherein:
 said steel slab has a composition further comprising one or more elements selected from the group consisting of:  
 Ni: from about 0.005 to 1.50 wt %,  
 Sn: from about 0.02 to 0.50 wt %,  
 Sb: from about 0.01 to 0.50 wt %,  
 Cu: from about 0.01 to 0.50 wt %,  
 Mo: from about 0.01 to 0.50 wt %, and  
 Cr: from about 0.01 to 0.50 wt %.