Cold rolled steel sheet and method for producing same

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

• C:
• 0.15% or more and 0.40% or less,
• Si:
• 0.50% or more and 4.00% or less,
• Mn:
• 1.00% or more and 4.00% or less,
• sol. Al:
• 0.001% or more and 2.000% or less,
• P:
• 0.020% or less,
• S:
• 0.020% or less,
• N:
• 0.010% or less,
• Ti:
• 0% or more and 0.200% or less,
• Nb:
• 0% or more and 0.200% or less,
• B:
• 0% or more and 0.010% or less,
• V:
• 0% or more and 1.00% or less,
• Cr:
• 0% or more and 1.00% or less,
• Mo:
• 0% or more and 1.00% or less,
• Cu:
• 0% or more and 1.00% or less,
• Co:
• 0% or more and 1.00% or less,
• W:
• 0% or more and 1.00% or less,
• Ni:
• 0% or more and 1.00% or less,
• Ca:
• 0% or more and 0.010% or less,
• Mg:
• 0% or more and 0.010% or less,
• REM:
• 0% or more and 0.010% or less,
• Zr:
• 0% or more and 0.010% or less, and
• balance:
• iron and impurities, and
• a metallic structure consisting of ferrite phases, hard second phases consisting of martensite phases and retained austenite phases, and remaining phases consisting of cementite phases and bainite phases, wherein
• an area ratio of the ferrite phases is 35% or more and 65% or less,
• an area ratio of the hard second phases is 35% or more and 65% or less,
• an area ratio of the remaining phases is 0% or more and 5% or less,
• 60% or more of the ferrite phases are recrystallized ferrite phases,
• an average crystal grain size defined by boundaries with differences in crystal orientations of 15 degrees or more is 5.0 μm or less,
• a maximum connecting rate of the hard second phases is 10% or more, and
• a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.