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

Abstract

A steel sheet for hot-pressing includes a specific chemical component composition. In the steel sheet, 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 3 nm or more. In the steel, a precipitated Ti amount and a total Ti amount in a steel satisfy the relationship of: Precipitated Ti amount (mass %)−3.4[N]≧0.5×[(total Ti amount (mass %))−3.4[N]] in which [N] indicates the content (mass %) of N in the steel. In the steel sheet, a ferrite fraction in a metal microstructure is 30 area % or more.

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 3 nm or more, 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, the method comprising:
 heating the steel sheet for hot-pressing as defined in  claim 1  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.;   starting press forming of the steel sheet which has been heated; and then   cooling the steel sheet 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 3 nm or more, and a carbon amount in the retained austenite is 0.50% or more. 
     
     
         5 . A method for manufacturing a press-formed article, the method comprising:
 wherein   using the steel sheet for hot-pressing as defined in  claim 1     dividing a heating region of the steel sheet into at least two regions;   heating one region of the divided regions at a temperature of Ac 3  transformation point or more and 950° C. or less;   heating another region of the divided regions 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,   starting press forming of both regions, and then   cooling the steel sheet 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 a carbon amount in the retained austenite is 0.50% or more. 
     
     
         7 . A method for manufacturing a press-formed article, the method comprising:
 heating the steel sheet for hot-pressing as defined in  claim 2  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.;   starting press forming of the steel sheet which has been heated; and then,   cooling the steel sheet 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.   
     
     
         8 . A press-formed article of a steel sheet having a chemical component composition as defined in  claim 2 , 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 3 nm or more, and a carbon amount in the retained austenite is 0.50% or more. 
     
     
         9 . A method for manufacturing a press-formed article, the method comprising:
 using the steel sheet for hot-pressing as defined in  claim 2 ;   dividing a heating region of the steel sheet into at least two regions;   heating one region of the divided regions at a temperature of Ac 3  transformation point or more and 950° C. or less;   heating another region of the divided regions 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,   starting press forming of both regions; and then,   cooling the steel sheet 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.   
     
     
         10 . A press-formed article of a steel sheet having a chemical component composition as defined in  claim 2 , 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 a carbon amount in the retained austenite is 0.50% or more.