Steel sheet, member, and methods for manufacturing same

Abstract

A steel sheet is disclosed. A related member, and methods for manufacturing them are also disclosed. The steel sheet has a specific chemical composition and a specific steel microstructure and is such that the total area fraction of quenched martensite and retained austenite each having an aspect ratio of 3 or less and an equivalent circular diameter of 2.0 μm or more is 20% or less relative to the total area fraction of quenched martensite and retained austenite, and the area fraction of a C-enriched region with a C concentration of 0.5 mass % or more (S c≥0.5 ) is 20% or less relative to the entire microstructure.

1 - 10 . (canceled) 
     
     
         11 . A steel sheet having a chemical composition comprising, in mass %,
 C: 0.05 to 0.20%,   Si: 0.40 to 1.50%,   Mn: 1.9 to 3.5%,   P: 0.02% or less,   S: 0.01% or less,   sol. Al: 1.00% or less, and   N: less than 0.015%,   the balance being iron and incidental impurities,   the steel sheet comprising a steel microstructure in which:   the area fraction of polygonal ferrite is 10% or more and 80% or less,   the total area fraction of upper bainite, tempered martensite, and lower bainite is 10% or more and 70% or less,   the volume fraction of retained austenite is 3% or more and 15% or less,   the area fraction of quenched martensite is 15% or less (including 0%), and   the steel microstructure further comprises a remaining microstructure,   the steel sheet being such that:   the total area fraction of quenched martensite and retained austenite each having an aspect ratio of 3 or less and an equivalent circular diameter of 2.0 μm or more is 20% or less relative to the total area fraction of quenched martensite and retained austenite, and   the area fraction of a C-enriched region with a C concentration of 0.5 mass % or more (S c≥0.5 ) is 20% or less relative to the entire microstructure.   
     
     
         12 . The steel sheet according to  claim 11 , wherein the chemical composition further comprises, in mass %, one or two or three selected from the following group A, group B and group C:
 group A:   one or two selected from:   Ti: 0.1% or less, and   B: 0.01% or less,   group B:   one, or two or more selected from:   Cu: 1% or less,   Ni: 1% or less,   Cr: 1% or less,   Mo: 0.5% or less,   V: 0.5% or less, and   Nb: 0.1% or less   group C:   one, or two or more selected from:   Mg: 0.0050% or less,   Ca: 0.0050% or less,   Sn: 0.1% or less,   Sb: 0.1% or less, and   REM: 0.0050% or less.   
     
     
         13 . The steel sheet according to  claim 11 , which has a galvanized layer on a surface. 
     
     
         14 . The steel sheet according to  claim 12 , which has a galvanized layer on a surface. 
     
     
         15 . A member obtained using the steel sheet described in  claim 11 . 
     
     
         16 . A member obtained using the steel sheet described in  claim 12 . 
     
     
         17 . A member obtained using the steel sheet described in  claim 13 . 
     
     
         18 . A member obtained using the steel sheet described in  claim 14 . 
     
     
         19 . A method for manufacturing a steel sheet, comprising hot rolling, pickling, and cold rolling a steel slab having the chemical composition described in  claim 11 , and annealing the resultant cold rolled steel sheet,
 the annealing comprising:   a holding step of heating the cold rolled steel sheet to an annealing temperature of 750 to 880° C. and holding the cold rolled steel sheet at the annealing temperature for 10 to 500 seconds;   a first cooling step of cooling the steel sheet to a first cooling stop temperature of 350 to 550° C. at a first average cooling rate in a range of temperatures from the annealing temperature to the first cooling stop temperature of 2 to 50° C./s;   a second cooling step of causing the steel sheet to reside at a residence temperature of 350 to 550° C. for 10 seconds or more and 60 seconds or less and cooling the steel sheet to a second cooling stop temperature of 150 to 360° C. at a second average cooling rate of 3 to 50° C./s; and   a third cooling step of cooling the steel sheet at a third average cooling rate in a range of temperatures from the second cooling stop temperature to 50° C. of 0.05 to 1.0° C./s.   
     
     
         20 . A method for manufacturing a steel sheet, comprising hot rolling, pickling, and cold rolling a steel slab having the chemical composition described in  claim 12 , and annealing the resultant cold rolled steel sheet,
 the annealing comprising:   a holding step of heating the cold rolled steel sheet to an annealing temperature of 750 to 880° C. and holding the cold rolled steel sheet at the annealing temperature for 10 to 500 seconds;   a first cooling step of cooling the steel sheet to a first cooling stop temperature of 350 to 550° C. at a first average cooling rate in a range of temperatures from the annealing temperature to the first cooling stop temperature of 2 to 50° C./s;   a second cooling step of causing the steel sheet to reside at a residence temperature of 350 to 550° C. for 10 seconds or more and 60 seconds or less and cooling the steel sheet to a second cooling stop temperature of 150 to 360° C. at a second average cooling rate of 3 to 50° C./s; and   a third cooling step of cooling the steel sheet at a third average cooling rate in a range of temperatures from the second cooling stop temperature to 50° C. of 0.05 to 1.0° C./s.   
     
     
         21 . The method for manufacturing a steel sheet according to  claim 19 , wherein the second cooling step causes the steel sheet to reside at a residence temperature of 350 to 550° C. for 10 seconds or more and 60 seconds or less while performing a hot-dip galvanizing treatment or a hot-dip galvannealing treatment on a surface of the steel sheet. 
     
     
         22 . The method for manufacturing a steel sheet according to  claim 20 , wherein the second cooling step causes the steel sheet to reside at a residence temperature of 350 to 550° C. for 10 seconds or more and 60 seconds or less while performing a hot-dip galvanizing treatment or a hot-dip galvannealing treatment on a surface of the steel sheet. 
     
     
         23 . The method for manufacturing a steel sheet according to  claim 19 , further comprising performing an electrogalvanizing treatment on a surface of the steel sheet after the annealing. 
     
     
         24 . The method for manufacturing a steel sheet according to  claim 20 , further comprising performing an electrogalvanizing treatment on a surface of the steel sheet after the annealing. 
     
     
         25 . A method for manufacturing a member, comprising a step of subjecting the steel sheet described in  claim 11  to at least one working of forming or joining to produce a member. 
     
     
         26 . A method for manufacturing a member, comprising a step of subjecting the steel sheet described in  claim 12  to at least one working of forming or joining to produce a member. 
     
     
         27 . A method for manufacturing a member, comprising a step of subjecting the steel sheet described in  claim 13  to at least one working of forming or joining to produce a member. 
     
     
         28 . A method for manufacturing a member, comprising a step of subjecting the steel sheet described in  claim 14  to at least one working of forming or joining to produce a member.