Electrical steel sheet and method of producing the same

Abstract

An electrical steel sheet includes a surface part in which a Si concentration in the steel sheet changes continuously from a high Si concentration to a low Si concentration in a thickness direction of the steel sheet from a surface of the steel sheet, as defined by a symmetry plane located at the center of the steel sheet in the thickness direction, a boundary part in which the Si concentration changes discontinuously, and an inner part in which the Si concentration does not change substantially in the thickness direction of the steel sheet, the inner part including the center of the steel sheet in the thickness direction, wherein the electrical steel sheet has a stress distribution such that an in-plane tensile stress is generated in the surface part and an in-plane compressive stress is generated in the inner part.

1 .- 6 . (canceled) 
     
     
         7 . An electrical steel sheet comprising:
 a surface part in which a Si concentration in the steel sheet changes continuously from a high Si concentration to a low Si concentration in a thickness direction of the steel sheet from a surface of the steel sheet, as defined by a symmetry plane located at the center of the steel sheet in the thickness direction,   a boundary part in which the Si concentration changes discontinuously, and   an inner part in which the Si concentration does not change substantially in the thickness direction of the steel sheet, the inner part including the center of the steel sheet in the thickness direction, wherein   the electrical steel sheet has a stress distribution such that an in-plane tensile stress is generated in the surface part and an in-plane compressive stress is generated in the inner part,   an average aspect ratio of crystal grains included in the surface part defined as a ratio of a dimension of the crystal grains in a direction parallel to the surface of the steel sheet to a dimension of the crystal grains in a direction (depth direction) perpendicular to the surface of the steel sheet, being 0.7 or more and 4.0 or less,   the average aspect ratio is the average of aspect ratios of 50 or more crystal grains and, when a crystal grain included in the surface part extends to the inner part beyond the boundary part, the dimension of the crystal grain in the direction (depth direction) perpendicular to the surface of the steel sheet includes a portion of the crystal grain which is included in the inner part.   
     
     
         8 . The electrical steel sheet according to  claim 7 , wherein the thickness of the surface part is 10% to 40% of the thickness of the steel sheet. 
     
     
         9 . The electrical steel sheet according to  claim 7 , wherein the average Si concentration in the surface part is 2.5% to 6.5% by mass and the average Si concentration in the inner part is 2.0% or less by mass. 
     
     
         10 . The electrical steel sheet according to  claim 8 , wherein the average Si concentration in the surface part is 2.5% to 6.5% by mass and the average Si concentration in the inner part is 2.0% or less by mass. 
     
     
         11 . The electrical steel sheet according to  claim 7 , wherein a tensile stress of 50 to 200 MPa is generated in the surface part in the direction parallel to the surface of the steel sheet, and a compressive stress of 50 to 200 MPa is generated in the inner part in the direction parallel to the surface of the steel sheet. 
     
     
         12 . The electrical steel sheet according to  claim 8 , wherein a tensile stress of 50 to 200 MPa is generated in the surface part in the direction parallel to the surface of the steel sheet, and a compressive stress of 50 to 200 MPa is generated in the inner part in the direction parallel to the surface of the steel sheet. 
     
     
         13 . The electrical steel sheet according to  claim 9 , wherein a tensile stress of 50 to 200 MPa is generated in the surface part in the direction parallel to the surface of the steel sheet, and a compressive stress of 50 to 200 MPa is generated in the inner part in the direction parallel to the surface of the steel sheet. 
     
     
         14 . The electrical steel sheet according to  claim 10 , wherein a tensile stress of 50 to 200 MPa is generated in the surface part in the direction parallel to the surface of the steel sheet, and a compressive stress of 50 to 200 MPa is generated in the inner part in the direction parallel to the surface of the steel sheet. 
     
     
         15 . The electrical steel sheet according to  claim 7 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         16 . The electrical steel sheet according to  claim 8 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         17 . The electrical steel sheet according to  claim 9 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         18 . The electrical steel sheet according to  claim 10 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         19 . The electrical steel sheet according to  claim 11 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         20 . The electrical steel sheet according to  claim 12 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         21 . The electrical steel sheet according to  claim 13 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         22 . The electrical steel sheet according to  claim 14 , the electrical steel sheet having a thickness of 0.03 to 0.5 mm. 
     
     
         23 . A method of producing an electrical steel sheet comprising:
 heating a steel sheet to 1100° C. to 1250° C. in a non-oxidizing atmosphere to transform the steel sheet into the austenite phase,   the steel sheet having a composition containing, by mass, C: 0.020% or less, Si: 0.15% to 2.0%, Mn: 0.05% to 2.00%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.01% or less, with the balance being Fe and inevitable impurities;   subsequently causing Si to penetrate a surface of the steel sheet at 1100° C. to 1250° C. in a non-oxidizing atmosphere containing 10 mol % or more and less than 45 mol % silicon tetrachloride to transform a surface layer of the steel sheet into the ferrite phase;   subsequently holding the steel sheet for a predetermined amount of time at 1100° C. to 1250° C. in a non-oxidizing atmosphere that does not contain Si until a thickness of a surface part that is in the ferrite phase reaches 10% to 40% of the thickness of the steel sheet, while maintaining the austenite phase in an inner part; and   subsequently cooling the steel sheet to 400° C. at an average cooling rate of 5 to 30° C./s.