Steel sheet, coated steel sheet, and methods for manufacturing same

Abstract

A steel sheet having TS of 590 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a predetermined steel microstructure, where an average aspect ratio of crystal grains of each phase (polygonal ferrite, martensite, and retained austenite) is 2.0 or more and 15.0 or less, wherein the polygonal ferrite has an average grain size of 6 μm or less, the martensite has an average grain size of 3 μm or less, the retained austenite has an average grain size of 3 μm or less, and a value obtained by dividing a Mn content in the retained austenite in mass % by a Mn content in the polygonal ferrite in mass % equals 2.0 or more.

1 - 6 . (canceled) 
     
     
         7 . A steel sheet comprising:
 a chemical composition containing, in mass %,
 C: 0.030% or more and 0.250% or less, 
 Si: 0.01% or more and 3.00% or less, 
 Mn: 2.60% or more and 4.20% or less, 
 P: 0.001% or more and 0.100% or less, 
 S: 0.0200% or less, 
 N: 0.0100% or less, and 
 Ti: 0.005% or more and 0.200% or less, and 
 optionally further containing, in mass %, at least one selected from the group consisting of
 Al: 0.01% or more and 2.00% or less, 
 Nb: 0.005% or more and 0.200% or less, 
 B: 0.0003% or more and 0.0050% or less, 
 Ni: 0.005% or more and 1.000% or less, 
 Cr: 0.005% or more and 1.000% or less, 
 V: 0.005% or more and 0.500% or less, 
 Mo: 0.005% or more and 1.000% or less, 
 Cu: 0.005% or more and 1.000% or less, 
 Sn: 0.002% or more and 0.200% or less, 
 Sb: 0.002% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.010% or less, 
 Ca: 0.0005% or more and 0.0050% or less, 
 Mg: 0.0005% or more and 0.0050% or less, and 
 REM: 0.0005% or more and 0.0050% or less, 
 
 with the balance consisting of Fe and inevitable impurities; and 
   a steel microstructure that contains, in area ratio,
 20% or more and 65% or less of polygonal ferrite, 
 8% or more of non-recrystallized ferrite, 
 and 5% or more and 25% or less of martensite, and 
 that contains, in volume fraction, 8% or more of retained austenite, where an average aspect ratio of crystal grains of each of the polygonal ferrite, the martensite, and the retained austenite is 2.0 or more and 15.0 or less, 
   wherein the polygonal ferrite has an average grain size of 6 μm or less, the martensite has an average grain size of 3 μm or less, the retained austenite has an average grain size of 3 μm or less, and a value obtained by dividing a Mn content in the retained austenite in mass % by a Mn content in the polygonal ferrite in mass % equals 2.0 or more.   
     
     
         8 . The steel sheet according to  claim 7 , wherein the retained austenite has a C content that satisfies the following formula in relation to the Mn content in the retained austenite:
   0.09*[Mn]−0.026−0.150≤[C]≤0.09*[Mn]−0.026+0.150
   where
 [C] is the C content in the retained austenite in mass %, and 
 [Mn] is the Mn content in the retained austenite in mass %. 
   
     
     
         9 . A coated steel sheet comprising:
 the steel sheet according to  claim 7 ; and   one selected from a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer, and an electrogalvanized layer provided on the steel sheet.   
     
     
         10 . A coated steel sheet comprising:
 the steel sheet according to  claim 8 ; and   one selected from a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer, and an electrogalvanized layer provided on the steel sheet.   
     
     
         11 . A method for manufacturing the steel sheet according to  claim 7 , the method comprising:
 (i) heating a steel slab having a chemical composition containing, in mass %,
 C: 0.030% or more and 0.250% or less, 
 Si: 0.01% or more and 3.00% or less, 
 Mn: 2.60% or more and 4.20% or less, 
 P: 0.001% or more and 0.100% or less, 
 S: 0.0200% or less, 
 N: 0.0100% or less, and 
 Ti: 0.005% or more and 0.200% or less, and 
 optionally further containing, in mass %, at least one selected from the group consisting of
 Al: 0.01% or more and 2.00% or less, 
 Nb: 0.005% or more and 0.200% or less, 
 B: 0.0003% or more and 0.0050% or less, 
 Ni: 0.005% or more and 1.000% or less, 
 Cr: 0.005% or more and 1.000% or less, 
 V: 0.005% or more and 0.500% or less, 
 Mo: 0.005% or more and 1.000% or less, 
 Cu: 0.005% or more and 1.000% or less, 
 Sn: 0.002% or more and 0.200% or less, 
 Sb: 0.002% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.010% or less, 
 Ca: 0.0005% or more and 0.0050% or less, 
 Mg: 0.0005% or more and 0.0050% or less, and 
 REM: 0.0005% or more and 0.0050% or less, 
 
 with the balance consisting of Fe and inevitable impurities; 
   (ii) hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;   (iii) coiling the steel sheet at 300° C. or higher and 750° C. or lower;   (iv) then subjecting the steel sheet to pickling to remove scales;   (v) retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+20° C.] to [Ac 1  transformation temperature+120° C.] for 600 s to 21,600 S;   (vi) optionally cold rolling the steel sheet at a rolling reduction of less than 30%; and   (vii) then retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+10° C.] to [Ac 1  transformation temperature+100° C.] for 20 s to 900 s and subsequently cooling the steel sheet.   
     
     
         12 . A method for manufacturing the steel sheet according to  claim 8 , the method comprising:
 (i) heating a steel slab having a chemical composition containing, in mass %,
 C: 0.030% or more and 0.250% or less, 
 Si: 0.01% or more and 3.00% or less, 
 Mn: 2.60% or more and 4.20% or less, 
 P: 0.001% or more and 0.100% or less, 
 S: 0.0200% or less, 
 N: 0.0100% or less, and 
 Ti: 0.005% or more and 0.200% or less, and 
 optionally further containing, in mass %, at least one selected from the group consisting of
 Al: 0.01% or more and 2.00% or less, 
 Nb: 0.005% or more and 0.200% or less, 
 B: 0.0003% or more and 0.0050% or less, 
 Ni: 0.005% or more and 1.000% or less, 
 Cr: 0.005% or more and 1.000% or less, 
 V: 0.005% or more and 0.500% or less, 
 Mo: 0.005% or more and 1.000% or less, 
 Cu: 0.005% or more and 1.000% or less, 
 Sn: 0.002% or more and 0.200% or less, 
 Sb: 0.002% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.010% or less, 
 Ca: 0.0005% or more and 0.0050% or less, 
 Mg: 0.0005% or more and 0.0050% or less, and 
 REM: 0.0005% or more and 0.0050% or less, 
 
 with the balance consisting of Fe and inevitable impurities; 
   (ii) hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;   (iii) coiling the steel sheet at 300° C. or higher and 750° C. or lower;   (iv) then subjecting the steel sheet to pickling to remove scales;   (v) retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+20° C.] to [Ac 1  transformation temperature+120° C.] for 600 s to 21,600 s;   (vi) optionally cold rolling the steel sheet at a rolling reduction of less than 30%; and   (vii) then retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+10° C.] to [Ac 1  transformation temperature+100° C.] for 20 s to 900 s and subsequently cooling the steel sheet.   
     
     
         13 . The method according to  claim 11 ,
 wherein a value obtained by dividing a volume fraction of the retained austenite after performing tensile working with an elongation value of 10% by a volume fraction of the retained austenite before the tensile working equals 0.3 or more.   
     
     
         14 . The method according to  claim 12 ,
 wherein a value obtained by dividing a volume fraction of the retained austenite after performing tensile working with an elongation value of 10% by a volume fraction of the retained austenite before the tensile working equals 0.3 or more.   
     
     
         15 . A method for manufacturing the coated steel sheet according to  claim 9 , comprising:
 (i) heating a steel slab having a chemical composition containing, in mass %,
 C: 0.030% or more and 0.250% or less, 
 Si: 0.01% or more and 3.00% or less, 
 Mn: 2.60% or more and 4.20% or less, 
 P: 0.001% or more and 0.100% or less, 
 S: 0.0200% or less, 
 N: 0.0100% or less, and 
 Ti: 0.005% or more and 0.200% or less, and 
 optionally further containing, in mass %, at least one selected from the group consisting of
 Al: 0.01% or more and 2.00% or less, 
 Nb: 0.005% or more and 0.200% or less, 
 B: 0.0003% or more and 0.0050% or less, 
 Ni: 0.005% or more and 1.000% or less, 
 Cr: 0.005% or more and 1.000% or less, 
 V: 0.005% or more and 0.500% or less, 
 Mo: 0.005% or more and 1.000% or less, 
 Cu: 0.005% or more and 1.000% or less, 
 Sn: 0.002% or more and 0.200% or less, 
 Sb: 0.002% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.010% or less, 
 Ca: 0.0005% or more and 0.0050% or less, 
 Mg: 0.0005% or more and 0.0050% or less, and 
 REM: 0.0005% or more and 0.0050% or less, with the balance consisting of Fe and inevitable impurities; 
 
   (ii) hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;   (iii) coiling the steel sheet at 300° C. or higher and 750° C. or lower;   (iv) then subjecting the steel sheet to pickling to remove scales;   (v) retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+20° C.] to [Ac 1  transformation temperature+120° C.] for 600 s to 21,600 s;   (vi) optionally cold rolling the steel sheet at a rolling reduction of less than 30%;   (vii) then retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+10° C.] to [Ac 1  transformation temperature+100° C.] for 20 s to 900 s and subsequently cooling the steel sheet; and   (viii) thereafter, either subjecting the steel sheet after cooling to one selected from hot-dip galvanizing treatment, hot-dip aluminum coating treatment, and electrogalvanizing treatment, or subjecting the steel sheet after cooling to hot-dip galvanizing treatment and then to alloying treatment at 450° C. or higher and 600° C. or lower.   
     
     
         16 . A method for manufacturing the coated steel sheet according to  claim 10 , comprising:
 (i) heating a steel slab having a chemical composition containing, in mass %,
 C: 0.030% or more and 0.250% or less, 
 Si: 0.01% or more and 3.00% or less, 
 Mn: 2.60% or more and 4.20% or less, 
 P: 0.001% or more and 0.100% or less, 
 S: 0.0200% or less, 
 N: 0.0100% or less, and 
 Ti: 0.005% or more and 0.200% or less, and 
 optionally further containing, in mass %, at least one selected from the group consisting of
 Al: 0.01% or more and 2.00% or less, 
 Nb: 0.005% or more and 0.200% or less, 
 B: 0.0003% or more and 0.0050% or less, 
 Ni: 0.005% or more and 1.000% or less, 
 Cr: 0.005% or more and 1.000% or less, 
 V: 0.005% or more and 0.500% or less, 
 Mo: 0.005% or more and 1.000% or less, 
 Cu: 0.005% or more and 1.000% or less, 
 Sn: 0.002% or more and 0.200% or less, 
 Sb: 0.002% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.010% or less, 
 Ca: 0.0005% or more and 0.0050% or less, 
 Mg: 0.0005% or more and 0.0050% or less, and 
 REM: 0.0005% or more and 0.0050% or less, 
 
 with the balance consisting of Fe and inevitable impurities; 
   (ii) hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet;   (iii) coiling the steel sheet at 300° C. or higher and 750° C. or lower;   (iv) then subjecting the steel sheet to pickling to remove scales;   (v) retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+20° C.] to [Ac 1  transformation temperature+120° C.] for 600 s to 21,600 s;   (vi) optionally cold rolling the steel sheet at a rolling reduction of less than 30%;   (vii) then retaining the steel sheet in a temperature range of [Ac 1  transformation temperature+10° C.] to [Ac 1  transformation temperature+100° C.] for 20 s to 900 s and subsequently cooling the steel sheet; and   (viii) thereafter, either subjecting the steel sheet after cooling to one selected from hot-dip galvanizing treatment, hot-dip aluminum coating treatment, and electrogalvanizing treatment, or subjecting the steel sheet after cooling to hot-dip galvanizing treatment and then to alloying treatment at 450° C. or higher and 600° C. or lower.