| 摘要 |
<p>Provided are a high strength steel sheet having a low C steel composition, a TS of 780 MPa or more, a TS x EL of 22000 MPa·% or more in addition to excellent hole expandability and stability of mechanical properties and a method for manufacturing the steel sheet. A high strength steel sheet having excellent formability and stability of mechanical properties has a composition including, by mass, C: 0.03% or more and 0.25% or less, Si: 0.4% or more and 2.5% or less, Mn: 3.5% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 2.5% or less, N: 0.008% or less, Si + Al: 1.0% or more, and the balance being Fe and inevitable impurities. In the steel microstructure, the area ratio of ferrite is 30% or more and 80% or less, the area ratio of martensite is 0% or more and 17% or less, the volume fraction of retained austenite is 8% or more, and the average grain size of retained austenite is 2 µm or less.</p> |
