Cold-rolled steel sheet and manufacturing method therefor

A cold-rolled steel sheet comprising:

• a chemical composition that contains, in mass %, C:
• 0.15% or more and 0.40% or less, Si:
• 1.5% or less, Mn:
• 0.9% to 1.7%, P:
• 0.03% or less, S:
• less than 0.0020%, sol.Al:
• 0.2% or less, N:
• less than 0.0055%, O:
• 0.0025% or less, at least one selected from the following groups (a) to (f):
• (a) B:
• 0.0002% or more and less than 0.0035%;
• (b) either or both of Nb:
• 0.002% to 0.08% and Ti:
• 0.002% to 0.12%;
• (c) either or both of Cu:
• 0.005% to 1% and Ni:
• 0.01% to 1%;
• (d) one or more selected from the group consisting of Cr:
• 0.01% to 1.0%, Mo:
• 0.01% or more and less than 0.3%, V:
• 0.003% to 0.5%, Zr:
• 0.005% to 0.2%, and W:
• 0.005% to 0.2%;
• (e) one or more selected from the group consisting of Ca:
• 0.0002% to 0.0030%, Ce:
• 0.0002% to 0.0030%, La:
• 0.0002% to 0.0030%, and Mg:
• 0.0002% to 0.0030%;
• and
• (f) either or both of Sb:
• 0.002% to 0.1% and Sn:
• 0.002% to 0.1%;
• and the balance consisting of Fe and incidental impurities, and that satisfies a relation expressed by:
• 5[% S]+[% N]<0.0115 (1),
• where [% S] and [% N] denote contents in mass % of S and N in steel, respectively;
• a microstructure in which
• tempered martensite and bainite are contained in a total area ratio of 95% or more and 100% or less with respect to a whole volume of the microstructure,
• a number of inclusion groups having a total length in a rolling direction of more than 120 μm is at most 0.8/mm2, the inclusion groups being formed by one or more inclusion particles, the one or more inclusion particles having a major axis length of 0.3 μm or more and extending and/or distributed in a dot-sequence manner along the rolling direction, and in the case of an inclusion group being formed by two or more inclusion particles, the two or more inclusion particles are spaced apart from one another by 30 μm or less,
• a number of carbides mainly composed of Fe that have an aspect ratio of 2.5 or less and a major axis length of 0.20 μm or more and 2 μm or less is at most 3,500/mm2,
• a number of carbides that are distributed in the tempered martensite and/or in the bainite and that have a diameter of 10 nm to 50 nm is at least 0.7×107/mm2, and
• prior γ grains have a mean grain size of 18 μm or less;
• a sheet thickness of 0.5 mm to 2.6 mm;
• and
• a tensile strength of 1,320 MPa or more,
• wherein an amount of sheet thickness variation in the rolling direction is 300 μm or less.