Cold rolled steel sheet and manufacturing method thereof

A cold rolled steel sheet comprising, by mass %:

• C:
• more than 0.150% to 0.300%;
• Si:
• 0.010% to 1.000%;
• Mn:
• 1.50% to 2.70%;
• P:
• 0.001% to 0.060%;
• S:
• 0.001% to 0.010%;
• N:
• 0.0005% to 0.0100%;
• and
• Al:
• 0.010% to 0.050%, and
• optionally one or more of:
• B:
• 0.0005% to 0.0020%;
• Mo:
• 0.01% to 0.50%;
• Cr:
• 0.01% to 0.50%;
• V:
• 0.001% to 0.100%;
• Ti:
• 0.001% to 0.100%;
• Nb:
• 0.001% to 0.050%;
• Ni:
• 0.01% to 1.00%;
• Cu:
• 0.01% to 1.00%;
• Ca:
• 0.0005% to 0.0050%;
• and
• REM:
• 0.0005% to 0.0050%, and
• a balance including Fe and unavoidable impurities,
• wherein, when an amount of C, an amount of Si and an amount of Mn are respectively represented by [C], [Si] and [Mn] in unit mass %, a relationship of the following formula 1 is satisfied,
• a metallographic structure contains, by area ratio, 40% to 90% of a ferrite and 10% to 60% of a martensite, and further contains one or more of 10% or less of a pearlite by area ratio, 5% or less of a retained austenite by volume ratio and 20% or less of a bainite by area ratio,
• a hardness of the martensite measured using a nanoindenter satisfies the following formulae 2a and 3a, and
• TS×λ representing a product of TS that is a tensile strength and λ that is a hole expansion ratio is 50000 MPa·% or more,
• (5×[Si]+[Mn])/[C]>10 (1)
• H20/H10<1.10 (2a)
• σHM0<20 (3a)
• here, the H10 represents an average hardness of the martensite at the surface part of the cold rolled steel sheet, the H20 represents an average hardness of the martensite at a center portion of a sheet thickness that occupies a ±100 μm range from a sheet thickness center of the cold rolled steel sheet in a thickness direction, and the σHM0 represents a variance of the hardness of the martensite present in the center portion of the sheet thickness.