High-strength steel sheet and method for producing same

A high-strength steel sheet comprising:

• a chemical composition consisting of, by mass %,
• C:
• 0.030% or more and 0.250% or less,
• Si:
• 0.01% or more and 3.00% or less,
• Mn:
• more than 4.20% and 6.00% or less,
• P:
• 0.001% or more and 0.100% or less,
• S:
• 0.0001% or more and 0.0200% or less,
• N:
• 0.0005% or more and 0.0100% or less, and
• Ti:
• 0.003% or more and 0.200% or less, and
• optionally at least one element selected from
• 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, or
• REM:
• 0.0005% or more and 0.0050% or less, and
• the balance being Fe and inevitable impurities, and
• a microstructure where
• ferrite is 15% or more and 55% or less in area ratio,
• martensite is 15% or more and 30% or less in area ratio, and
• retained austenite is 12% or more in volume fraction, wherein
• an average grain size of the ferrite is 4.0 μm or less,
• an average grain size of the martensite is 2.0 μm or less,
• an average grain size of the retained austenite is 2.0 μm or less,
• an average aspect ratio of crystal grain of each of the ferrite, the martensite and the retained austenite is more than 2.2 and 15.0 or less,
• a value obtained by dividing a Mn content (mass %) in the retained austenite by a Mn content (mass %) in the ferrite is 2.0 or more, and
• the high-strength steel sheet has a tensile strength of 980 MPa or more and a yield ratio of less than 68%.