Steel sheet for hot stamped member and method of production of same

Abstract

A steel sheet for obtaining a member which is excellent in fatigue characteristics equal to ordinary high strength steel sheet of the same strength even if applying the hot stamping process and a method of production of the same are provided. Steel sheet for a hot stamped member which includes composition which contains, by mass %, C: 0.15 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable impurities, and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, wherein that a standard error of Vicker's hardness at a position of 20 μm from the steel sheet surface in the sheet thickness direction is 20 or less. This steel sheet is produced by a recrystallization-annealing step of a first stage of heating a cold rolled steel sheet, which is obtained by hot rolling steel containing the above composition and then cold rolling it, by an average heating rate of 8 to 25° C./sec from room temperature to 600 to 700° C., then a second stage of heating by an average heating rate of 1 to 7° C./sec to 720 to 820° C.

1 . Steel sheet for a hot stamped member which includes composition which contains, by mass %,
 C: 0.15 to 0.35%,   Si: 0.01 to 1.0%,   Mn: 0.3 to 2.3%,   Al: 0.01 to 0.5%, and   a balance of Fe and unavoidable impurities, and limit the impurities to   P: 0.03% or less,   S: 0.02% or less, and   N: 0.1% or less,   wherein a standard deviation of Vicker's hardness at a position of 20 μm from the steel sheet surface in the sheet thickness direction is 20 or less.   
     
     
         2 . The steel sheet for a hot stamped member as set forth in  claim 1  which further contains, by mass %, one or more of elements selected from
 Cr: 0.01 to 2.0%, 
 Ti: 0.001 to 0.5%, 
 Nb: 0.001 to 0.5% 
 B: 0.0005 to 0.01%, 
 Mo: 0.01 to 1.0% 
 W: 0.01 to 0.5%, 
 V: 0.01 to 0.5%, 
 Cu: 0.01 to 1.0%, and 
 Ni: 0.01 to 5.0%. 
 
     
     
         3 . The steel sheet for a hot stamped member as set forth in  claim 1  which has on the surface of said steel sheet one of a 5 μm to 50 μm thick Al plating layer, a 5 μm to 30 μm thick galvanized layer, or a 5 μm to 45 μm thick Zn—Fe alloy layer. 
     
     
         4 . A method of production of steel sheet for a hot stamped member comprising recrystallization-annealing cold rolled steel sheet which includes composition which contains, by mass %,
 C: 0.15 to 0.35%,   Si: 0.01 to 1.0%,   Mn: 0.3 to 2.3%,   Al: 0.01 to 0.5%, and   a balance of Fe and unavoidable impurities, and limit the impurities to   P: 0.03% or less,   S: 0.02% or less, and   N: 0.1% or less,   in which step, including   a first stage of heating by an average heating rate of 8 to 25° C./sec from room temperature to a temperature M (° C.) and   then a second stage of heating by an average heating rate of 1 to 7° C./sec to a temperature S (° C.),   wherein the temperature M (° C.) is 600 to 700 (° C.) and the temperature S (° C.) is 720 to 820 (° C.).   
     
     
         5 . The method of production of steel sheet for a hot stamped member as set forth in  claim 4  wherein said steel further contains, by mass %, one or more of
 Cr: 0.01 to 2.0%, 
 Ti: 0.001 to 0.5%, 
 Nb: 0.001 to 0.5% 
 B:  0 . 0005  to 0.01%, 
 Mo: 0.01 to 1.0% 
 W: 0.01 to 0.5%, 
 V: 0.01 to 0.5%, 
 Cu: 0.01 to 1.0%, and 
 Ni: 0.01 to 5.0%. 
 
     
     
         6 . The method of production of steel sheet for a hot stamped member as set forth in  claim 5  wherein a hot rolling rate in said hot rolling step is 60 to 90%, while a cold rolling rate of said cold rolling step is 30 to 90%. 
     
     
         7 . The method of production of steel sheet for a hot stamped member as set forth in  claim 4  which further includes, after said recrystallization-annealing step, a step of dipping said steel sheet in an Al bath to form an Al plating layer on the surface. 
     
     
         8 . The method of production of steel sheet for a hot stamped member as set forth in  claim 4  which further includes, after said recrystallization annealing step, a step of dipping said steel sheet in a Zn bath to form a galvanized layer on the surface. 
     
     
         9 . The method of production of steel sheet for a hot stamped member as set forth in  claim 4  which further includes, after said recrystallization-annealing step, a step of dipping said steel sheet in a Zn bath to form a galvanized layer on the surface, then further heating to 600° C. or less to form a Zn—Fe alloy layer on said surface.