Steel sheet for hot stamping use, method of production of same, and method of production of high strength part

Abstract

The present invention has as its object the provision of steel sheet for hot stamping use which is excellent in part strength after hot stamping and delayed fracture resistance comprised of large C content high strength steel sheet in which effective hydrogen traps are formed in the steel material. The steel sheet of the present invention solves this problem by forming Fe—Mn-based composite oxides in the steel sheet and trapping hydrogen at the interfaces of the composite oxides and matrix steel and in the voids around the composite oxides. Specifically, it provides steel sheet for hot stamping use which is comprised of chemical ingredients which contain, by mass %, C: 0.05 to 0.40%, Si: 0.02% or less, Mn: 0.1 to 3%, S: 0.02% or less, P: 0.03% or less, Al: 0.005% or less, Ti: 0.01% or less, N: 0.01% or less, one or both of Cr and Mo in a total of 0.005 to 1%, and O: 0.003 to 0.03% and which have a balance of Fe and unavoidable impurities and which contains average diameter 0.1 to 15 μm Fe—Mn-based composite oxide particles dispersed in the steel sheet or furthermore has crushed voids around the composite oxide particles, a method of production of the same, and a method of production of a hot stamped high strength part.

1 . A method of production of steel sheet for hot stamping use comprising hot rolling a slab of chemical ingredients in which rough rolling the slab by a rolling rate of 70% or more and final rolling the slab by a rolling rate of 70% or more, wherein the chemical ingredients contain, by mass %, 
       C: 0.05 to 0.40%, 
       Si: 0.001 to 0.02%, 
       Mn: 0.1 to 3%, 
       Al: 0.0002 to 0.005%, 
       Ti: 0.0005 to 0.01%, 
       O: 0.003 to 0.03%,
 one or more of Cr and Mo in a total of 0.005 to 2%, and 
 a balance of Fe and unavoidable impurities. 
 
     
     
         2 . The method of production of steel sheet for hot stamping use as set forth in  claim 1 , wherein the chemical ingredients further contain, by mass %, an ingredient(s) which is included in one or more groups among the three groups of (a) to (c): 
       (a) B: 0.0005 to 0.01%; 
       (b) one or more of Nb, V, W, and Co in a total of 0.005 to 1%; and 
       (c) one or more of Ni and Cu in a total of 0.005 to 2%. 
     
     
         3 . The method of production of steel sheet for hot stamping use as set forth in  claim 1 , further comprising pickling the hot rolled steel sheet which was obtained by hot rolling and cold rolling the steel sheet by a rolling rate of 30% or more. 
     
     
         4 . The method of production of steel sheet for hot stamping use as set forth in  claim 2 , further comprising pickling the hot rolled steel sheet which was obtained by hot rolling and cold rolling the steel sheet by a rolling rate of 30% or more. 
     
     
         5 . The method of production of steel sheet for hot stamping use as set forth in  claim 3 , further comprising annealing the cold rolled steel sheet which was obtained by cold rolling. 
     
     
         6 . The method of production of steel sheet for hot stamping use as set forth in  claim 4 , further comprising annealing the cold rolled steel sheet which was obtained by cold rolling. 
     
     
         7 . A method of production of a high strength part using a steel sheet for hot stamping use comprising heating the steel sheet to a temperature of austenite region of the Ac 3  or higher, then starting to form the steel sheet by a die, and cooling the steel sheet in the die after forming to quench, wherein
 the steel sheet is comprised of chemical ingredients which contain, by mass %,   
       C: 0.05 to 0.40%, 
       Si: 0.001 to 0.02%, 
       Mn: 0.1 to 3%, 
       Al: 0.0002 to 0.005%, 
       Ti: 0.0005 to 0.01%, 
       O: 0.003 to 0.03%,
 one or more of Cr and Mo in a total of 0.005 to 2%, and 
 a balance of Fe and unavoidable impurities, and 
 wherein the steel sheet contains average diameter 0.1 to 15 μm Fe—Mn-based composite oxide particles dispersed in the steel sheet. 
 
     
     
         8 . The method of production of a high strength part using a steel sheet for hot stamping use as set forth in  claim 7 , wherein the chemical ingredients further contain, by mass %, an ingredient(s) which is included in one or more groups among the three groups of (a) to (c): 
       (a) B: 0.0005 to 0.01%; 
       (b) one or more of Nb, V, W, and Co in a total of 0.005 to 1%; and 
       (c) one or more of Ni and Cu in a total of 0.005 to 2%.