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. 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 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, 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, 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.