Steel sheet and method for producing the same

A steel sheet having a chemical composition comprising, by mass %:

• C:
• 0.06% to 0.25%;
• Si:
• 0.6% to 2.5%;
• Mn:
• 2.3% to 3.5%;
• P:
• 0.02% or less;
• S:
• 0.01% or less;
• sol. Al:
• less than 0.50%;
• N:
• less than 0.015%;
• and
• the balance being Fe and incidental impurities,
• wherein the steel sheet has a steel microstructure comprising ferrite:
• 5% or less by area percentage, at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ:
• in a range of 95% to 100% by area percentage, and retained γ:
• in a range of 5% to 20% by volume percentage,
• retained γUB with a grain width in a range of 0.25 to 0.60 μm, a grain length in a range of 1.0 to 15 μm, and an aspect ratio in a range of 3.1 to 25 has an area percentage SγUB in a range of 0.2% to 7.0%,
• retained γLB with a grain width in a range of 0.08 to 0.24 μm, a grain length in a range of 0.6 to 15 μm, and an aspect ratio in a range of 4 to 40 has a distribution number NγLB in a range of 10 to 120 per 100 μm2,
• at least one of (i) fresh martensite with an equivalent circular grain diameter in a range of 0.5 μm or more and less than 1.3 μm and at least one of an aspect ratio of 3 or less and (ii) retained γ grains with an equivalent circular grain diameter in a range of 0.5 μm or more and less than 1.3 μm and an aspect ratio of 3 or less has a total area percentage SγFine in the range of 1% to 10%, and
• at least one of (i) fresh martensite with an equivalent circular grain diameter in a range of 1.5 to 20 μm and an aspect ratio of 3 or less and (ii) retained γ grains with an equivalent circular grain diameter in a range of 1.5 to 20 μm and an aspect ratio of 3 or less have a total area percentage SγBlock of 5% or less, including 0%.