High-strength cold-rolled steel sheet and method for manufacturing the same

A high-strength cold-rolled steel sheet having a chemical composition comprising, by mass %:

• C:
• 0.15% to 0.25%;
• Si:
• 1.2% to 2.5%;
• Mn:
• 2.1% to 3.5%;
• P:
• 0.05% or less;
• S:
• 0.005% or less;
• Al:
• 0.01% to 0.08%;
• N:
• 0.010% or less;
• Ti:
• 0.002% to 0.050%;
• B:
• 0.0002% to 0.0100%;
• and
• the balance being Fe and inevitable impurities,
• wherein the cold-rolled steel sheet has a microstructure including (i) ferrite having an average crystal grain diameter of 2 μm or less in an amount in a range of 10% to 25% in terms of volume fraction, (ii) retained austenite in an amount in a range of 5% to 20% in terms of volume fraction, (iii) martensite having an average crystal grain diameter of 2 μm or less in an amount in a range of 5% to 15% in terms of volume fraction, and (iv) the balance being a multi-phase structure including bainite and tempered martensite, the bainite and tempered martensite having an average crystal grain diameter of 5 μm or less, the tempered martensite being present in the microstructure in an amount in a range of 20% to 60% in terms of volume fraction, the bainite being present in the microstructure in an amount in a range of 10% to 40% in terms of volume fraction, and
• relational expression (1) is satisfied:
• 0.35≤V2/V1≤0.75 (1)
• where V1 is a volume fraction of phases which are different from ferrite and V2 is a volume fraction of tempered martensite.