High-strength cold-rolled steel sheet, high-strength coated or plated steel sheet, method of producing high-strength cold-rolled steel sheet, method of producing high-strength coated or plated steel sheet, and automotive part

A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more as measured in accordance with JIS Z 2201, comprising:

• a chemical composition that consists of, in mass %,
• C:
• 0.06% or more and 0.15% or less,
• Si:
• 0.10% or more and 1.8% or less,
• Mn:
• 2.00% or more and 3.50% or less,
• P:
• 0.050% or less,
• S:
• 0.0050% or less,
• N:
• 0.0060% or less,
• Al:
• 0.010% or more and 1.0% or less,
• Ti:
• 0.005% or more and 0.075% or less,
• Nb:
• 0.005% or more and 0.075% or less, and
• B:
• 0.0002% or more and 0.0040% or less, and
• optionally at least one element selected from the group consisting of:
• V:
• 0.200% or less,
• Cr:
• 0.20% or less,
• Mo:
• 0.20% or less,
• Cu:
• 0.30% or less,
• Ni:
• 0.30% or less,
• Sb:
• 0.100% or less,
• Sn:
• 0.100% or less,
• Ca:
• 0.0050% or less,
• Mg:
• 0.0050% or less,
• REM:
• 0.0050% or less,
• Ta:
• 0.100% or less,
• W:
• 0.500% or less,
• Zr:
• 0.0200% or less, and
• Co:
• 0.100% or less,
• with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1, where [mol % N] and [mol % Ti] are respectively a content of N and a content of Ti in steel in mol %;
• and
• a steel microstructure in which:
• an area fraction of ferrite is 30% or more and 60% or less;
• a total area fraction of tempered martensite and bainite is 35% or more and 65% or less;
• an area fraction of quenched martensite is 15% or less;
• an area fraction of retained austenite is 1% or more and 10% or less;
• an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 40% or less, where [% Mn] is a content of Mn in the steel in mass %;
• a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more;
• an area fraction of a residual microstructure is less than 3%;
• and
• an average grain size of the low-Mn ferrite is 10 μm or less.