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

Abstract

A steel sheet for hot pressing use according to the present invention has a specified chemical component composition, 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, the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1) shown below, and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more. Precipitated Ti amount (mass %)−3.4[N]>0.5×[total Ti amount (mass %)−3.4[N]]  (1) (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.