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 steel sheet formed by a hot press-forming method, and having a metallic structure that contains ferrite at 30% to 80% by area, bainitic ferrite at lower than 30% by area (not including 0% by area), martensite at 30% by area or lower (not including 0% by area), and retained austenite at 3% to 20% by area, 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 steel sheet formed by a hot press-forming method, and having a metallic structure that contains ferrite at 30% to 80% by area, bainitic ferrite at lower than 30% by area (not including 0% by area), martensite at 30% by area or lower (not including 0% by area), and retained austenite at 3% to 20% by area. 
     
     
         2 . The hot press-formed product according to  claim 1 , having the following chemical element composition:
 C at 0.1% to 0.3% (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%; 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, B at 0.01% or lower (not including 0%) and Ti at 0.1% or lower (not including 0%). 
     
     
         4 . 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, Cr, and Mo at 1% or lower (not including 0%) in total. 
     
     
         5 . 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. 
     
     
         6 . A process for producing a hot press-formed product as set forth in  claim 1 , comprising:
 heating a hot-rolled steel sheet having a metallic structure that contains ferrite at 50% by area or higher, or heating a cold-rolled steel sheet at a reduction of 30% or higher, to a temperature not lower than Ac 1  transformation point and not higher than (Ac 1  transformation point×0.3+Ac 3  transformation point×0.7); and then   starting the forming of the hot-rolled steel sheet or the cold-rolled 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 (bainite transformation starting temperature Bs−100° C.).   
     
     
         7 . 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.;   cooling the thin steel sheet to a temperature not higher than 700° C. and not lower than 500° C. at an average cooling rate of 10° C./sec or lower; 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 (bainite transformation starting temperature Bs−100° C.).   
     
     
         8 . The process according to  claim 6 , wherein the forming finishing temperature is controlled in a temperature range of not higher than (bainite transformation starting temperature Bs−100° C.) and not lower than martensite transformation starting temperature Ms, in which temperature range the steel sheet is retained for 10 seconds or longer, followed by the forming. 
     
     
         9 . 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 is a hot-rolled steel sheet having a metallic structure that contains ferrite at 50% by area or higher, or a cold-rolled steel sheet at a reduction of 30% or higher.