Hot-pressing steel plate, press-molded article, and method for manufacturing press-molded article

Abstract

Provided is a hot-pressing steel plate useful for obtaining a press-molded article having excellent anti-softening characteristics in heat-affected zones (HAZ) while attaining a press-molded article that can achieve a high level of balance between high strength and stretchability if uniform characteristics within the molded article are required, and a high level of balance between high strength and stretchability in respective regions if regions corresponding to shock-resistant portions and energy-absorbing portions are required within one molded article; molding and processing prior to hot-pressing being facilitated by the hot-pressing steel plate having a prescribed chemical composition, having the equivalent circular diameter of Ti-containing deposits included in the steel plate be 30 nm or less with the average equivalent circular diameter of the Ti-containing deposits being 6 nm or less, having the deposited Ti amount and the total Ti amount within the steel satisfy a prescribed relation, and having a metal structure with a proportion of ferrite of 30% by area or greater.

1 . A steel sheet for hot-pressing, comprising:
 C: from 0.15 to 0.5% (mass %; hereinafter, the same applies to the chemical component composition),   Si: from 0.2 to 3%,   Mn: from 0.5 to 3%,   P: 0.05% or less (exclusive of 0%),   S: 0.05% or less (exclusive of 0%),   Al: from 0.01 to 1%,   B: from 0.0002 to 0.01%,   Ti: equal to or more than 3.4[N]+0.01% and equal to or less than 3.4[N]+0.1% (wherein [N] indicates a content (mass %) of N), and   N: from 0.001 to 0.01%, with the remainder being iron and unavoidable impurities, wherein   an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less, a precipitated Ti amount and a total Ti amount in a steel satisfy a relationship of the following formula (1), and a ferrite fraction in a metal microstructure is 30 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] indicates the content (mass %) of N in the steel).   
     
     
         2 . The steel sheet for hot-pressing according to  claim 1 , comprising, as the other element(s), at least one of the following (a) to (c):
 (a) one or more kinds selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in total;   (b) one or more kinds selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in total; and   (c) one or more kinds selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in total.   
     
     
         3 . A method for manufacturing a press-formed article, wherein the steel sheet for hot-pressing as defined in  claim 1  is heated at a temperature equal to or more than Ac 1  transformation point+20° C. and equal to or less than Ac 3  transformation point-20° C., and then press forming of the steel sheet is started, and the steel sheet is cooled to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature Bs while ensuring an average cooling rate of 20° C./sec or more in a mold during forming and after a completion of forming. 
     
     
         4 . A press-formed article of a steel sheet having a chemical component composition as defined in  claim 1 , wherein a metal microstructure of the press-formed article includes retained austenite: from 3 to 20 area %, ferrite: from 30 to 80 area %, bainitic ferrite: less than 30 area % (exclusive of 0 area %), and martensite: 31 area % or less (exclusive of 0 area %), and an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article is 10 nm or less, and a precipitated Ti amount and a total Ti amount in a steel satisfy the relationship of the following formula (1):
   Precipitated Ti amount (mass %)−3.4[N]<0.5×[(total Ti amount (mass %))−3.4[N]]   (1)
   (in the formula (1), [N] indicates the content (mass %) of N in the steel).   
     
     
         5 . A method for manufacturing a press-formed article, wherein the steel sheet for hot-pressing as defined in  claim 1  is used, a heating region of the steel sheet is divided into at least two regions, one region of them is heated at a temperature of Ac 3  transformation point or more and 950° C. or less, another region of them is heated at a temperature equal to or more than Ac 1  transformation point+20° C. and equal to or less than Ac 3  transformation point−20° C., and then press forming of both regions is started, and the steel sheet is cooled to a temperature equal to or less than a martensite transformation starting temperature Ms while ensuring an average cooling rate of 20° C./sec or more in a mold in both of the regions during forming and after a completion of forming. 
     
     
         6 . A press-formed article of a steel sheet having a chemical component composition as defined in  claim 1 , which has a first region having a metal microstructure including retained austenite: from 3 to 20 area % and martensite: 80 area % or more and a second region having a metal microstructure including retained austenite: from 3 to 20 area %, ferrite: from 30 to 80 area %, bainitic ferrite: less than 30 area % (exclusive of 0 area %), and martensite: 31 area % or less (exclusive of 0 area %), and an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in a steel of the second region is 10 nm or less, and a precipitated Ti amount and a total Ti amount in the steel satisfy the relationship of the following formula (1):
   Precipitated Ti amount (mass %)−3.4[N]<0.5×[(total Ti amount (mass %))−3.4[N]]   (1)
   (in the formula (1), [N] indicates the content (mass %) of N in the steel).