Hot press molding and manufacturing method therefor

Abstract

A hot press molding including: a first forming region exhibiting a metal structure, which contains 80-97 area % of martensite and 3-20 area % of retained austenite, respectively, and in which the residual structure is 5 area % or less; and a second forming region exhibiting a metal structure, which contains 70-97 area % of bainitic ferrite, 27 area % or less of martensite, and 3-20 area % of retained austenite, respectively, and in which the residual structure is 5 area % or less. As a result, hot press moldings, which have at least a region corresponding to a shock-resistant area and a region corresponding to an energy-absorbing area in a single molding and in which a high level of balancing of high strength with elongation according to the respective region can be achieved, are provided without using a welding method.

1 . A hot press molded article formed by hot press molding of a thin steel plate, comprising:
 a first molding region exhibiting a metal structure which contains 80-97 area % of martensite and 3-20 area % of retained austenite and which has a residual structure at 5 area % or less; and   a second molding region exhibiting a metal structure which contains 70-97 area % of bainitic ferrite, 27 area % or less of martensite, and 3-20 area % of retained austenite and which has a residual structure at 5 area % or less.   
     
     
         2 . The hot press molded article according to  claim 1 , wherein
 the first and second molding regions have an identical chemical component composition, and   steel of each component region contains, in units of mass %,
 0.15-0.3% of C, 
 0.5-3% of Si, 
 0.5-2% of Mn, 
 0.05% or less of P, 
 0.05% or less of S, 
 0.01-0.1% of Al, 
 0.01-1% of Cr, 
 0.0002-0.01% of B, 
 [N]×4-0.1% of Ti, and 
 0.001-0.01% of N, where 0% is not inclusive for the P and the S, and [N] denotes an N content in units of %, and 
   the steel of each component region has a residual consisting of iron and an inevitable impurity.   
     
     
         3 . The hot press molded article according to  claim 2 , wherein
 the steel further contains, as other element, one or more selected from a group consisting of Cu, Ni, and Mo in a total amount of 1% or less, where 0% is not inclusive.   
     
     
         4 . The hot press molded article according to  claim 2 , wherein
 the steel further contains, as other element, at least one of V or Nb in a total amount of 0.1% or less, where 0% is not inclusive.   
     
     
         5 . A method for producing the hot press molded article according to  claim 1  by forming a thin steel plate so as to divide the thin steel plate into a plurality of regions including at least first and second molding regions, the method comprising:
 after the thin steel plate is heated to a temperature of an Ac3 transformation point or higher and 1000° C. or lower, starting cooling at an average cooling rate of 20° C./sec or higher and molding by pressing at least the first and second molding regions together with a die; and 
 terminating, in the first molding region, the molding at equal to or lower than a temperature lower than a martensite transformation start temperature by 50° C., and performing, in the second molding region, the cooling to a temperature range of equal to or lower than a temperature lower than a bainite transformation start temperature by 100° C. and equal to or higher than the martensite transformation start temperature and terminating the molding after a lapse of a stay time of 10 seconds or longer within the temperature range.