Steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product

Abstract

(In the formula (1), [N] represents the content (mass %) of N in the steel.)

1 . A steel sheet, comprising, in mass %, with respect to a chemical component composition:
 C: 0.15-0.5%;   Si: 0.2-3%;   Mn: 0.5-3%;   P: 0.05% or less, exclusive of 0%;   S: 0.05% or less, exclusive of 0%;   Al: 0.01-1%;   B: 0.0002-0.01%;   Ti: 3.4[N]+0.01% or more and 3.4[N]+0.1% or less, wherein [N] represents N content in mass %; and   N: 0.001-0.01% respectively, with a remainder comprising iron and inevitable impurities, wherein   some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 3 nm or more, a precipitated Ti amount and a total Ti amount in the steel fulfill a relationship of formula (1), and a sum total of a fraction of bainite and a fraction of martensite in a metal microstructure is 80 area % or more:
   precipitated Ti amount (mass %)−3.4[ N]> 0.5×[(total Ti amount (mass %))−3.4[ N]]   (1)
 
   wherein [N] represents the content (mass %) of N in the steel.   
     
     
         2 . The steel sheet according to  claim 1 , further comprising at least one of (a)-(c) below as other elements:
 (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less, exclusive of 0%, in total;   (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less exclusive of 0%, in total; and   (c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less, exclusive of 0%, in total.   
     
     
         3 . A method for manufacturing a press-formed product, comprising:
 hot pressing with a steel sheet according to  claim 1 ;   heating the steel sheet to a temperature of Ac 1  transformation point+20° C. or above and Ac 3  transformation point−20° C. or below;   thereafter starting press forming; and   executing cooling to a temperature or below, the temperature being lower than a bainite transformation starting temperature Bs by 100° C., while securing an average cooling rate of 20° C./s or more within a tool during forming and after completion of forming.   
     
     
         4 . A press-formed product obtained by the method according to  claim 3 , wherein the metal microstructure comprises retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, and an amount of carbon in the retained austenite is 0.60% or more. 
     
     
         5 . A method for manufacturing a press-formed product, comprising:
 hot pressing with the steel sheet according to  claim 1 ;   dividing a heating region of the steel sheet into two regions;   heating one region thereof to a temperature of Ac 3  transformation point or above and 950° C. or below;   heating the other region to a temperature of Ac 1  transformation point+20° C. or above and Ac 3  transformation point−20° C. or below;   thereafter starting press forming; and   executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing an average cooling rate of 20° C./s or more within a tool during forming and after completion of forming.   
     
     
         6 . A press-formed product obtained by the method according to  claim 5 , comprising a first region whose metal microstructure comprises retained austenite: 3-20 area % and martensite: 80 area % or more, and a second region whose metal microstructure comprises retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area % with an amount of carbon in the retained austenite being 0.60% or more.