| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet with a composite structure having an excellent balance in tensile strength and formability and having an excellent fatigue life. SOLUTION: The steel sheet has a composition comprising, by mass, 0.01 to 0.1% C, 0.5 to 2.0% Si, 0.5 to 2.0% Mn, ≤0.03% P, ≤0.003% S, 0.005 to 0.04% Al, ≤0.006% N, 0.01 to 0.15% Ti, and the balance Fe with inevitable impurities, and has a microstructure where, in an area fraction, 90 to <99% is ferrite as the main phases and the second phase is made of martensite, in the cross-section orthogonal to the sheet width direction, the maximum value of the length in the rolling direction of the second phase is ≤20 μm, further, the density of the ones with an equivalent circle diameter of ≥1.0 μm is ≤10,000 pieces/mm<SP>2</SP>in the second phase, and the total in the rolling direction of inclusions in which the length in the rolling direction is ≥30 μm in the cross-section is ≤0.25 mm per mm<SP>2</SP>. COPYRIGHT: (C)2011,JPO&INPIT |
