Hot press-formed product, process for producing same, and thin steel sheet for hot press forming

Abstract

There is provided a hot press-formed product, including a thin steel sheet formed by a hot press-forming method, and having a metallic structure that contains martensite at 80% to 97% by area and retained austenite at 3% to 20% by area, the remainder structure of which is at 5% by area or lower, whereby balance between strength and elongation can be controlled in a proper range and high ductility can be achieved.

1 . A hot press-formed product, comprising a thin steel sheet formed by a hot press-forming method, and having a metallic structure that contains martensite at 80% to 97% by area and retained austenite at 3% to 20% by area, the remainder structure of which is at 5% by area or lower. 
     
     
         2 . The hot press-formed product according to  claim 1 , having the following chemical element composition:
 C at 0.15% to 0.35% (where “%” means “% by mass”, and the same applies to the below with respect to the chemical element composition);   Si at 0.5% to 3%;   Mn at 0.5% to 2%;   P at 0.05% or lower (not including 0%);   S at 0.05% or lower (not including 0%);   Al at 0.01% to 0.1%;   Cr at 0.01% to 1%;   B at 0.0002% to 0.01%;   Ti at (N content)×4% to 0.1%; and   N at 0.001% to 0.01%,   
       and the remainder consisting of iron and unavoidable impurities. 
     
     
         3 . The hot press-formed product according to  claim 2 , further comprising, as additional elements, one or more selected from the group consisting of Cu, Ni, and Mo at 1% or lower (not including 0%) in total. 
     
     
         4 . The hot press-formed product according to  claim 2 , further comprising, as additional elements, V and/or Nb at 0.1% or lower (not including 0%) in total. 
     
     
         5 . A process for producing a hot press-formed product as set forth in  claim 1 , comprising:
 heating a thin steel sheet to a temperature not lower than Ac 3  transformation point and not higher than 1000° C.; and then   starting the forming of the thin steel sheet with a press tool to produce the hot press-formed product, during which forming an average cooling rate of 20° C./sec or higher is kept in the press tool, and which forming is finished at a temperature not higher than (martensite transformation starting temperature Ms point−50° C.).   
     
     
         6 . A thin steel sheet for hot press forming, which is intended for use in producing a hot press-formed product as set forth in  claim 1 , and which has the following chemical element composition:
 C at 0.15% to 0.35%;   Si at 0.5% to 3%;   Mn at 0.5% to 2%;   P at 0.05% or lower (not including 0%);   S at 0.05% or lower (not including 0%);   Al at 0.01% to 0.1%;   Cr at 0.01% to 1%;   B at 0.0002% to 0.01%;   Ti at (N content)×4% to 0.1%; and   N at 0.001% to 0.01%,   
       and the remainder consisting of iron and unavoidable impurities. 
     
     
         7 . The thin steel sheet for hot press forming according to  claim 6 , further comprising, as additional elements, one or more selected from the group consisting of Cu, Ni, and Mo at 1% or lower (not including 0%) in total. 
     
     
         8 . The hot press-formed product according to  claim 6 , further comprising, as additional elements, V and/or Nb at 0.1% or lower (not including 0%) in total.