Steel sheet and method for producing the same

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

• C:
• 0.13% to 0.40%,
• Si:
• 1.5% or less,
• Mn:
• 1.8% to 4%,
• P:
• 0.02% or less,
• S:
• less than 0.0010%,
• sol. Al:
• 0.2% or less,
• N:
• less than 0.0060%,
• B:
• 0.0003% or more and less than 0.0035%, and
• O:
• less than 0.0020%,
• at least one of Nb:
• 0.002% or more and less than 0.035%, and Ti:
• 0.002% or more and less than 0.040%, and
• the balance being Fe and inevitable impurities,
• wherein the steel sheet has a microstructure including (i) martensite and bainite, a total area fraction of the martensite and the bainite to an entirety of the microstructure being in a range of more than 90% and 100% or less, (ii) prior-γ grains having an average grain size of 6 to 15 μm, (iii) inclusion clusters A having a major axis of 100 μm or more, each of the inclusion clusters A being constituted by one or more inclusion particles, the inclusion particles having a major axis of 0.3 μm or more, and, in the case where the inclusion clusters are constituted by two or more inclusion particles, a shortest distance between the inclusion particles is 30 μm or less, and inclusion clusters B having a major axis of 20 μm or more and less than 100 μm, each of the inclusion clusters B being constituted by one or more inclusion particles, the inclusion particles having a major axis of 0.3 μm or more, and, in the case where the inclusion clusters are constituted by two or more inclusion particles, a shortest distance between the inclusion particles is 10 μm or less, the content of the inclusion clusters A in the microstructure being 2 clusters/mm2 or less, and the content of the inclusion clusters B in the microstructure being 5 clusters/mm2 or less, and (iv) carbide particles including Fe as a main constituent, the carbide particles having an aspect ratio of 2.0 or less and a major axis of 0.30 μm or more and 2 μm or less, and the content of the carbide particles in the microstructure being 4000 particles/mm2 or less,
• Formula (1) and Formula (2) are both satisfied:
• [% Ti]+[% Nb]>0.007 (1)
• [% Ti]×[% Nb]2<7.5×10−6 (2)
• where [% Nb] and [% Ti] represent the contents (mass %) of Nb and Ti, respectively, and
• the steel has a thickness of 0.5 to 2.6 mm and a tensile strength of 1320 MPa or more.