Steel plate for hot forming and manufacturing method of hot press formed steel member

Abstract

Provided is a method for manufacturing a steel member through hot press forming using a steel sheet that contains a comparatively large amount of Si. The method allows hot press forming to be carried out satisfactorily by stably and reliably suppressing shedding/peeling of oxide scale during hot press forming, even without restricting hot press forming conditions to a narrow range. The steel sheet includes, in mass ratio, C: 0.15% to 0.35%, Si: 1.0% to 3.0%, Mn: 1.0% to 3.0%, Al: more than 0% up to 0.10%, Ti: ([N]×48/14) % to 0.10% (where [N] denotes the amount of N in the steel sheet), B: 5 ppm to 50 ppm, P: more than 0% to less than 0.015%, S: more than 0% up to 0.010%, and N: more than 0% up to 0.010%, the balance being iron and unavoidable impurities. The average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass % or higher.

1 . A steel sheet comprising, in mass ratio:
 C: 0.15% to 0.35%;   Si: 1.0% to 3.0%;   Mn: 1.0% to 3.0%;   Al: more than 0% up to 0.10%;   Ti: ([N]×48/14) % to 0.10% where [N] denotes the amount of N in the steel sheet;   B: 5 ppm to 50 ppm;   P: more than 0% to less than 0.015%;   S: more than 0% up to 0.010%; and   N: more than 0% up to 0.010%,   the steel sheet further comprising iron,   wherein an average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass % or higher.   
     
     
         2 . The steel sheet of  claim 1 , wherein the average oxygen concentration from the outermost surface of the steel sheet down to a depth of 10 μm in the sheet thickness direction is 0.85 mass % or higher. 
     
     
         3 . The steel sheet of  claim 1 ,
 further comprising as other elements, in mass ratio:   one or more elements selected from the group consisting of   Cr: more than 0% up to 1.0%;   Mo: more than 0% up to 0.5%; and   one or more from among V, Nb and W: more than 0% up to 0.5%.   
     
     
         4 . The steel sheet of  claim 2 ,
 further comprising as other elements, in mass ratio:   one or more elements selected from the group consisting of   Cr: more than 0% up to 1.0%;   Mo: more than 0% up to 0.5%; and   one or more from among V, Nb and W: more than 0% up to 0.5%.   
     
     
         5 . A method for manufacturing a hot press formed steel member, the method comprising heating and hot press forming the steel sheet of  claim 1 ,
 wherein   the heating is performed at a heating temperature of 1100° C. or lower,   a dwell time in an oxidizing atmosphere and at 800° C. or higher is set to 40 seconds or less within a time period from start of heating until completion of hot press forming, and   a starting temperature of the hot press forming is set to 600° C. or higher.