High strength steel sheet and method of producing same

Abstract

A high strength steel sheet containing a large amount of Mn which has bendability suitable as a material for auto part use and a method of producing the same are provided. A high strength steel sheet comprising a center part in a sheet thickness and a softened surface layer part on one surface or both surfaces of the center part in the sheet thickness, characterized in that, the center part in the sheet thickness has an average Mn concentration of greater than 4.0 mass % and less than 10.0 mass %, each softened surface layer part has a thickness of 0.1% to 30% of the sheet thickness, and the softened surface layer part has an average Mn concentration of 2.5 mass % or less, a recrystallization rate of 90% or more, and recrystallized structures with an average crystal grain size of 0.1 μm or more and 40 μm or less.

1 . A high strength steel sheet comprising
 a center part in a sheet thickness and   a softened surface layer part on one surface or both surfaces of the center part in the sheet thickness,   characterized in that,   the center part in the sheet thickness has an average Mn concentration of greater than 4.0 mass % and less than 10.0 mass %,   each softened surface layer part has a thickness of 0.1% to 30% of the sheet thickness, and   the softened surface layer part has an average Mn concentration of 2.5 mass % or less, a recrystallization rate of 90% or more, and recrystallized structures with an average crystal grain size of 0.1 μm or more and 40 μm or less.   
     
     
         2 . The high strength steel sheet according to  claim 1 , characterized in that
 the center part in the sheet thickness contains, by mass %,   C: greater than 0.05% and less than 0.80%,   Si: 0.001% or more and less than 3.50%,   Mn: greater than 4.0% and less than 10.0%,   P: 0.10% or less,   S: 0.010% or less,   sol. Al: 0.001% or more and less than 3.00%,   N: less than 0.050%, and   a balance of iron and unavoidable impurities.   
     
     
         3 . The high strength steel sheet according to  claim 2 , characterized in that the center part in the sheet thickness further contains, by mass %, at least one selected from a group consisting of
 Cr: 0.01% or more and 2.00% or less,   Mo: 0.01% or more and 2.00% or less,   Cu: 0.01% or more and 2.00% or less, and   Ni: 0.01% or more and 2.00% or less.   
     
     
         4 . The high strength steel sheet according to  claim 2  [[or 3]], characterized in that
 the center part in the sheet thickness further contains, by mass %, at least one selected from a group consisting of 
 Ti: 0.005% or more and 0.30% or less, 
 Nb: 0.005% or more and 0.30% or less, 
 V: 0.005% or more and 0.30% or less, and 
 W: 0.005% or more and 0.30% or less. 
 
     
     
         5 . The high strength steel sheet according to any one of  claims 2  to  4   claim 2 , characterized in that the center part in the sheet thickness further contains, by mass %, at least one selected from a group consisting of
 B: 0.0001% or more and 0.010% or less, 
 Ca: 0.0001% or more and 0.010% or less, 
 Mg: 0.0001% or more and 0.010% or less, 
 Zr: 0.0001% or more and 0.010% or less, and 
 REM: 0.0001% or more and 0.010% or less. 
 
     
     
         6 . The high strength steel sheet according to  claim 2 , characterized in that the center part in the sheet thickness further contains, by mass %, at least one selected from a group consisting of
 Sb: 0.0005% or more and 0.050% or less,   Sn: 0.0005% or more and 0.050% or less, and   Bi: 0.0005% or more and 0.050% or less.   
     
     
         7 . The high strength steel sheet according to  claim 2 , characterized in that an amount of C in the softened surface layer part is 0.9 time or less of an amount of C in the center part in the sheet thickness. 
     
     
         8 . The high strength steel sheet according to  claim 3 , characterized in that a total of an amount of Cr and an amount of Mo in the softened surface layer part is 0.9 time or less of a total of an amount of Cr and an amount of Mo in the center part in the sheet thickness. 
     
     
         9 . The high strength steel sheet according to  claim 3 , characterized in that a total of an amount of Cu and an amount of Ni in the softened surface layer part is 0.9 time or less of a total of an amount of Cu and an amount of Ni in the center part in the sheet thickness. 
     
     
         10 . The high strength steel sheet according to  claim 4 , characterized in that a total of an amount of Ti and an amount of Nb in the softened surface layer part is 0.9 time or less of a total of an amount of Ti and an amount of Nb in the center part in the sheet thickness. 
     
     
         11 . The high strength steel sheet according to  claim 4 , characterized in that a total of an amount of V and an amount of W in the softened surface layer part is 0.9 time or less of a total of an amount of V and an amount of W in the center part in the sheet thickness. 
     
     
         12 . The high strength steel sheet according to  claim 5 , characterized in that an amount of B in the softened surface layer part is 0.9 time or less of an amount of B in the center part in the sheet thickness. 
     
     
         13 . The high strength steel sheet according to  claim 1 , characterized in that a surface of the softened surface layer part further contains a hot dip galvanized layer, a hot dip galvannealed layer, or an electrogalvanized layer. 
     
     
         14 . A method of producing the high strength steel sheet according to  claim 1 , characterized in that the method comprises:
 superposing a softened surface layer part-use steel sheet constituting the softened surface layer part on one surface or both surfaces of a base material steel sheet constituting the center part in the sheet thickness to form a multilayer steel sheet,   heating the multilayer steel sheet to 1080° C. or more and 1300° C. or less and hot rolling the heated multilayer steel sheet under conditions of a finish rolling start temperature of 800° C. or more and 1000° C. or less,   cooling the hot rolled multilayer steel sheet to 500° C. or more and 700° C. or less within 2 seconds after the end of the finish rolling,   cooling the multilayer steel sheet down to a temperature of 500° C. or more and 700° C. or less, and then holding the multilayer steel sheet for 3 seconds or more,   pickling the multilayer steel sheet held for 3 seconds or more at a temperature of 500° C. or more and 700° C. or less, and then cold rolling the multilayer steel sheet by a rolling reduction of 20% or more and 70% or less, and   holding the cold rolled multilayer steel sheet at a temperature of 600° C. or more and 750° C. or less for 5 seconds or more, and then cooling the cold rolled multilayer steel sheet.   
     
     
         15 . The method of producing the high strength steel sheet according to  claim 14 , characterized by coiling the multilayer steel sheet held for 3 seconds or more at a temperature of 500° C. or more and 700° C. or less at a coiling temperature of 600° C. or less. 
     
     
         16 . The method of producing the high strength steel sheet according to  claim 15 , characterized by holding the coiled multilayer steel sheet at a temperature of 300° C. or more and 550° C. or less before the cold rolling so as to temper the coiled multilayer steel sheet.