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 6 nm or less, 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, by mass %, iron and:
 C: from 0.15-0.5%;   Si: from 0.2-3%;   Mn: from 0.5-3%;   P: from 0.05% or less (exclusive of 0%);   S: from 0.05% or less (exclusive of 0%);   Al: from 0.01-1%;   B: from 0.0002-0.01%;   Ti: from 3.4[N]+0.002% to 3.4[N]+0.1% ([N] expresses N content (mass %)), and   N: from 0.001-0.01% and   Ti-containing precipitates said precipitates have an equivalent circle diameter of 30 nm or less, and an average equivalent circle diameter of 6 nm or less, and wherein   the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1), 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)
 
   where [N] represents the content (mass %) of N in the steel.   
     
     
         2 . The steel sheet according to  claim 1  further comprising:
 (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/or 
 (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:
 heating the steel sheet of  claim 1  to a temperature of Ac 1  transformation point+20° C. or above and Ac 3  transformation point−20° C. or below;   press-forming the heated steel sheet; and   holding the steel sheet at the bottom dead point and cooling to a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., with an average cooling rate of 20° C./s or more thereby producing the press-formed product.   
     
     
         4 . A press-formed product of a steel sheet, said sheet comprising iron and
 C: from 0.15-0.5%;   Si: from 0.2-3%;   Mn: from 0.5-3%;   P: from 0.05% or less (exclusive of 0%);   S: from 0.05% or less (exclusive of 0%);   Al: from 0.01-1%;   B: from 0.0002-0.01%;   Ti: from 3.4[N]+0.002% to 3.4[N]+0.1% ([N] expresses N content (mass %)), and   N: from 0.001-0.01%,   wherein a metal microstructure within the press-formed product comprises retained austenite from 3-20 area %, annealed martensite and/or annealed bainite from 30-87 area %, and martensite as quenched from 10-67 area %, Ti-containing precipitates, said precipitates have an equivalent circle diameter of 30 nm or less, and an average equivalent circle diameter of 10 nm or less,   and the amount of precipitated Ti and the total amount of Ti in the formed product fulfill the relationship represented by formula (1):
   Precipitated Ti amount (mass %)−3.4[N]0.5×[(total Ti amount (mass %))−3.4[N]]  (1)
 
   where [N] represents the content (mass %) of N in the steel.   
     
     
         5 . A method for manufacturing a press-formed product comprising:
 subdividing a heating region of the steel sheet from  claim 1  into at least two regions thereby producing divided regions,   heating one divided region to a temperature of from Ac 3  transformation point to 950° C.,   heating another divided region not identical to the previous divided region to a temperature of from Ac 1  transformation point+20° C. to Ac 3  transformation point−20° C.,   press forming the heated subdivided regions; and   holding the subdivided regions at the bottom dead point and cooling to a temperature of martensite transformation starting temperature Ms or below at an average cooling rate of 20° C./s or more thereby producing the press-formed product.   
     
     
         6 . A press-formed product of a steel sheet, said sheet comprising iron and
 C: from 0.15-0.5%;   Si: from 0.2-3%;   Mn: from 0.5-3%;   P: from 0.05% or less (exclusive of 0%);   S: from 0.05% or less (exclusive of 0%);   Al: from 0.01-1%;   B: from 0.0002-0.01%;   Ti: from 3.4[N]+0.002% to 3.4[N]+0.1% ([N] expresses N content (mass %)), and   N: from 0.001-0.01%,   wherein   the press-formed product comprises a first region whose metal microstructure comprises retained austenite of from 3-20 area % and martensite as quenched of 80 area % or more, and a second region whose metal microstructure comprises retained austenite of from 3-20 area %, annealed martensite and/or annealed bainite of from 30-87 area %; and martensite as quenched of from 10-67 area %, and   some Ti-containing precipitates comprising the steel of the second region have an equivalent circle diameter of 30 nm or less, and an average equivalent circle diameter of 10 nm or less, and   the precipitated Ti amount and the total Ti amount in the formed product fulfill the relationship represented by formula (1):
   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.