High-strength steel sheet and production method for same, and production method for high-strength galvanized steel sheet

1. A high-strength steel sheet comprising:

a chemical composition containing, in mass %, C: 0.08% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 3.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and optionally at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Ti: 0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% or less, B: 0.0001% or more and 0.0050% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.0020% or more and 0.2000% or less, Sn: 0.0020% or more and 0.2000% or less, Ta: 0.0010% or more and 0.1000% or less, Ca: 0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and 0.0050% or less, and REM: 0.0003% or more and 0.0050% or less, and the balance consisting of Fe and incidental impurities; and

a steel microstructure that contains, by area, 20% or more and 50% or less of ferrite, 5% or more and 25% or less of bainitic ferrite, and 5% or more and 20% or less of martensite, and by volume, 10% or more of retained austenite, and by area, 10% or less of other phases including tempered martensite, pearlite, and cementite, wherein

the retained austenite has a mean grain size of 2 μm or less,

a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, and

the retained austenite has a mean free path of 1.2 μm or less, and

wherein the high-strength steel sheet has a tensile strength (TS) of 780 MPa or more and satisfies:

total elongation (EL) ≥34% for TS 780 MPa grade, EL ≥27% for TS 980 MPa grade, and EL ≥23% for TS 1180 MPa grade,

fatigue limit strength ≥400 MPa, and

fatigue ratio ≥0.40.

2. A production method for a high-strength steel sheet, the method comprising:

heating a steel slab having the chemical composition as recited in claim 1 to 1100° C. or higher and 1300° C. or lower;

hot rolling the steel slab with a finisher delivery temperature of 800° C. or higher and 1000° C. or lower to obtain a steel sheet;

coiling the steel sheet at a mean coiling temperature of 450° C. or higher and 700° C. or lower;

subjecting the steel sheet to pickling treatment;

optionally, retaining the steel sheet at a temperature of 450° C. or higher and Ac 1 transformation temperature or lower for 900 s or more and 36000 s or less,

cold rolling the steel sheet at a rolling reduction of 30% or more;

subjecting the steel sheet to first annealing treatment whereby the steel sheet is heated to a temperature of 820° C. or higher and 950° C. or lower;

cooling the steel sheet to a first cooling stop temperature at or below Ms at a mean cooling rate to 500° C. of 15° C./s or higher;

subjecting the steel sheet to second annealing treatment whereby the steel sheet is reheated to a temperature of 740° C. or higher and 840° C. or lower;

cooling the steel sheet to a temperature in a second cooling stop temperature range of 300° C. to 550° C. at a mean cooling rate of 1° C./s or higher and 10° C./s or lower; and

retaining the steel sheet at the second cooling stop temperature range for 10 s or more, to produce the high-strength steel sheet as recited in claim 1 .

3. The production method for a high-strength steel sheet according to claim 2 , the method further comprising after the retaining at the second cooling stop temperature range for 10 s or more in the second annealing treatment, subjecting the steel sheet to third annealing treatment at a temperature of 100° C. or higher and 300° C. or lower.

4. A production method for a high-strength galvanized steel sheet, the method comprising subjecting the high-strength steel sheet as recited in claim 1 to galvanizing treatment.